Insights | ProBuild Management

18 July 2026

How Much Should a House Extension Cost?

"How much will my extension cost?"

It's one of the first questions we're asked, and one of the hardest to answer honestly.

The truthful answer is always the same: it depends on what you're building, where you're building it and how you're going to build it. Without understanding those three things, any figure you receive is little more than an educated guess.

Most people are hoping for a simple answer, but construction rarely works like that.

There Is No Perfect Project

One of the simplest ways I've ever seen construction explained comes from The Housebuilder's Bible by Mark Brinkley. He described what many people know as the builder's triangle: Quick. Cheap. Good.

Over the years I've adapted that to something I think better reflects the conversations we have with clients today: Cost. Speed. Quality.

Every project sits somewhere within that triangle, and you can usually optimise two of those priorities, but rarely all three. If your priority is completing the project quickly and to an exceptional standard, it probably won't be the cheapest option. If your priority is achieving the lowest possible cost while maintaining quality, you'll often need more flexibility on programme.

Understanding those trade-offs at the beginning is one of the biggest steps towards a successful project.

Is Cost Per Square Metre Actually Useful?

People often ask what the average cost per square metre is. The honest answer is that it can be useful, but only as a very rough guide, and the range is far wider than most people realise.

Location plays a part. Building in Central London is very different to building in the Home Counties, and different again to building in the Midlands or the North. The biggest influences, however, are usually specification and complexity. Two extensions of exactly the same size can have dramatically different costs because one uses standard construction methods while the other includes large structural openings, bespoke glazing, premium finishes or more challenging structural work.

Square metre rates should only ever be treated as an early budgeting tool, never mistaken for the cost of your project.

You're Not Just Building an Extension

One of the biggest misconceptions is that an extension is simply an extra room added onto the side or rear of a house. In reality, you're often rebuilding part of your home. The new space has to connect properly with the existing building, which usually involves structural alterations, drainage, heating, electrics, insulation, finishes, and making old and new work together as one.

Then something else often happens. A project that starts as a kitchen extension quickly becomes a ground floor renovation. The new kitchen makes the existing flooring look tired. Once the heating system is being altered, upgrading the rest of it suddenly makes sense. If electricians are already on site, improving lighting elsewhere becomes far easier. Before long, the project has naturally grown.

That isn't poor planning. It's simply how many domestic projects evolve once homeowners begin looking at their house as a whole, rather than just the new space.

The Biggest Mistake We See

One of the most expensive mistakes homeowners make isn't choosing the wrong builder. It's going out to tender before they understand what the project should actually cost.

Many people obtain three or four quotations and compare the totals. The difficulty is that quotations are only as useful as the person reviewing them. Unless somebody has already prepared a proper cost plan, it's extremely difficult to know whether every contractor has priced exactly the same scope of work. You may not be comparing like for like at all.

I often compare it to buying a car. Very few people would walk into a dealership without first understanding roughly what that vehicle should cost. Construction should be no different. Knowing what your project is likely to cost before asking contractors to price it gives you a benchmark against which every quotation can be measured.

Why Can Two Similar Extensions Cost £100,000 Apart?

People are often surprised that two extensions of a similar size can differ enormously in price. There are many reasons.

Ground conditions may require different foundations. One project may include a £7,000 kitchen while another includes a £60,000 bespoke installation. The amount of glazing, structural steel, specialist brickwork and internal finishes can all have a significant impact.

The procurement route also matters. A larger contractor may offer more management, greater programme certainty and stronger quality assurance, and those benefits inevitably carry higher overheads than a smaller contractor or a self-managed project.

Headline figures rarely tell the whole story.

Spend Money Where It Matters

If there's one piece of advice I'd give, it's this: spend your money on the things you're unlikely to change. Invest in the quality of the building itself, the fabric, the insulation, the windows and doors, the heating system and the electrical infrastructure. These are the things you'll still appreciate twenty years from now.

By comparison, kitchens, bathrooms and decoration often change far sooner than people imagine.

Better Decisions Cost Less Than Better Quotes

One of the biggest opportunities to save money comes before the architect has finished drawing. Architects are experts in design, just as contractors are experts in construction. Both bring valuable perspectives, and the best outcomes happen when those perspectives come together early.

A contractor with commercial and construction experience can help shape the brief before expensive design decisions become fixed. That might mean recommending a different construction method, suggesting an alternative product, or simply identifying a detail that achieves the same result more efficiently. Those conversations are far easier before a project goes out to tender than afterwards.

A Final Thought

For most families, an extension is one of the biggest investments they'll ever make. It's exciting, but it's also complex. There are regulations to navigate, budgets to manage and countless decisions to make.

Even the most organised homeowner benefits from experienced guidance, because the cost of making poor decisions early is usually far greater than the cost of making good ones.

The question isn't simply "How much does an extension cost?" A better question is "How can I achieve the outcome I want within the budget I have?" In our experience, that's where successful projects begin.

15 June 2026

Construction Management vs Principal Contractor: Which Is Right for Your Project?

One of the first questions many clients ask is whether they should appoint a construction manager or a principal contractor. It's a fair question, but it's often the wrong place to start.

The better question is: which procurement route is right for your project?

Neither option is inherently better than the other. They simply suit different types of projects, different clients and different levels of involvement. Understanding the difference before you begin can save a significant amount of money, time and frustration later on.

What Is Construction Management?

In simple terms, construction management means employing a professional team to manage the project on your behalf, rather than employing a contractor to deliver the entire build.

The construction manager coordinates the project, procures trade contractors, manages the programme, oversees health and safety, coordinates information and keeps the project moving. The individual trade contractors are appointed directly by you, and the construction manager is paid a management fee for delivering the service.

Many people imagine a construction manager simply sits on site supervising trades. In reality, the role is far broader than that. It involves procurement, sequencing, package management, contractor vetting, programming, quality control and constant coordination between designers, consultants and specialist contractors.

It is a highly involved process and works best when the client also wants to be actively involved in the project.

What Is a Principal Contractor?

A principal contractor takes responsibility for delivering the agreed scope of works for an agreed contract sum.

Rather than appointing individual trade contractors yourself, you appoint one company. That contractor then manages their own workforce and subcontractors, coordinates the programme and accepts contractual responsibility for delivering the project.

Their price will include their management costs, overheads and profit, along with an allowance for the commercial risks they are taking on.

For many clients, the biggest advantage is simplicity. Instead of coordinating numerous appointments and decisions, there is a single point of responsibility throughout the construction process.

Is Construction Management Cheaper?

This is probably the biggest misconception surrounding construction management. Many people assume it must be cheaper because you're removing the contractor's overhead and profit, and on paper, that often appears to be true.

Instead of paying a principal contractor's commercial margin, you're paying a construction management fee while appointing trade contractors directly. However, construction costs aren't determined solely by margins. If the programme extends by several months, if numerous design changes are made, or if trade packages end up costing more than anticipated, those savings can quickly disappear, and you may find yourself paying construction management fees for far longer than expected while absorbing additional costs that a principal contractor may have managed differently.

Equally, an experienced principal contractor may secure better pricing through established relationships with subcontractors and suppliers, savings that aren't always obvious when comparing quotations.

For that reason, neither route is automatically the cheaper option. The final cost depends on how the project is managed, how decisions are made and how successfully risk is controlled throughout the build.

Which Clients Suit Construction Management?

Construction management tends to work particularly well in two situations.

The first is on highly complex projects involving numerous specialist trades and a significant amount of ongoing coordination. High-end residential homes, luxury refurbishments, hotels and large commercial developments often benefit from this approach because they require constant management of specialist contractors and evolving design information.

The second is for clients who genuinely want to be involved. If you already have trusted trades you'd like to use, expect to make regular design changes or simply want complete visibility over procurement and costs as the project develops, construction management offers a level of flexibility that is difficult to achieve under a traditional fixed-price contract.

When Is a Principal Contractor the Better Option?

Not every client wants to make dozens of decisions every week; many simply want confidence that somebody is taking responsibility for delivering the project.

If your priority is greater cost certainty, a single point of accountability and a more turnkey approach, appointing a principal contractor is often the more appropriate procurement route. This doesn't remove every risk from construction (no procurement route can), but it does transfer much of the day-to-day responsibility to the contractor through a formal contract.

Transparency Isn't Exclusive to Construction Management

One of the biggest myths is that construction management is transparent while principal contracting isn't. That simply isn't true.

Modern principal contractors can be extremely transparent in how they present costs, value variations and administer contracts. Where projects have been properly cost planned and professionally reviewed before construction begins, clients should understand exactly what they're buying and how costs are managed throughout the project.

From our perspective, transparency is the gateway to trust. The more clearly a contractor explains how costs are built up and how decisions are made, the easier it becomes for everyone to work together throughout the build.

No Two Contractors Are the Same

One mistake we see regularly is assuming that all principal contractors operate in exactly the same way. They don't.

Some employ large in-house construction teams. Others rely almost entirely on subcontractors. Some build competitive tender prices before recovering profit through variations. Others include realistic allowances from the outset to reduce commercial conflict later.

Construction managers also differ in how they procure work, manage projects and communicate with clients.

The procurement route is only part of the decision; choosing the right team is just as important.

So Which Approach Would We Recommend?

It depends entirely on the project.

There are projects where we actively recommend construction management because it offers the flexibility and level of involvement the client genuinely wants. In some high-end residential projects, trying to fix every element of the build under a traditional principal contractor arrangement would introduce unnecessary risk and ultimately increase cost.

Equally, there are many projects where appointing a principal contractor provides exactly the certainty and accountability the client is looking for. Our role is to help clients choose the procurement route that best suits their project, not to steer everyone towards one option.

Successful construction isn't about following the same process every time. It's about selecting the right approach before the first brick is laid.

10 May 2026

Understanding Value Engineering: Reducing Cost Without Compromising Quality

One of the most misunderstood terms in construction is value engineering. For many people, it sounds like a polite way of saying "make it cheaper," but good value engineering is something quite different.

It's about reviewing a design before and during construction to find better ways of achieving the same outcome. Sometimes that reduces cost. Sometimes it saves time. Sometimes it improves buildability. Often, it achieves all three.

At its heart, value engineering is about making better decisions.

What Is Value Engineering?

The simplest way to think about value engineering is this: it's the process of analysing a project and asking whether there is a more efficient way of achieving the same result. That might involve changing a construction method, selecting a different product, simplifying a structural detail or choosing an alternative installation sequence.

The objective is to maximise value, not reduce quality. Good value engineering considers much more than cost. It also considers programme, buildability, long-term performance and the overall objectives of the project.

Value Engineering Isn't the Same as Cutting Costs

This is where many people get confused. Simply cutting costs usually means removing scope or specifying cheaper finishes. Value engineering is different: the aim is to maintain the quality and intent of the design while finding smarter ways to deliver it.

For example, changing a complicated construction detail to a simpler one may reduce labour, shorten the programme and produce exactly the same finished result. The client receives the same outcome, but with less time, less complexity and often less cost. That's value engineering.

Buildability Matters More Than Many People Realise

One of the biggest factors affecting construction cost is something called buildability. Put simply, the easier something is to build, the less it generally costs. Complex details require more labour, more coordination and more opportunity for errors, and simplifying those details often improves both cost and programme without changing the finished appearance.

Take a three-storey house with a complicated traditional roof structure. By redesigning that roof using attic trusses supplied by a specialist manufacturer, you may reduce the amount of structural steel, speed up installation and even improve the layout of the rooms within the roof space. The project becomes simpler to build, quicker to deliver and potentially more cost-effective, all while achieving the same overall objective. That's the type of thinking good value engineering encourages.

The Earlier You Start, the Better

The greatest opportunities for value engineering exist before construction begins. Once contracts have been signed and work is underway, every significant change becomes more expensive to implement. That's why we always encourage clients to review their designs thoroughly before appointing a principal contractor, since this is the point where relatively small changes on paper can prevent significant additional costs on site.

That doesn't mean value engineering stops once construction starts. Experienced contractors and construction managers regularly identify opportunities during the build where alternative methods or products can improve efficiency, since some opportunities simply aren't visible until work begins. The key is maintaining an open conversation throughout the project.

Where Clients Accidentally Overspend

Overspending doesn't always happen because clients choose expensive products. Often, it happens for two reasons.

The first is that nobody questions the original specification. Architects quite rightly design around performance, planning and aesthetics, but there are often multiple products or construction methods capable of achieving the same result, and without someone reviewing those decisions commercially, clients can end up paying for specifications they didn't even know existed.

The second is finishes. Many people naturally associate higher prices with better quality. Sometimes that's true. Often it isn't. Excellent kitchens, flooring, tiles and sanitaryware are available at a wide range of price points, and understanding where additional spending genuinely improves the finished project, and where it simply pays for a brand name, is an important part of delivering value.

One Area Where Clients Shouldn't Try to Save Money

If there is one area where we would advise caution, it's pre-construction. Trying to minimise professional fees at the beginning of a project can often become one of the most expensive decisions made during the entire build.

A good architect, quantity surveyor, planning consultant or contract administrator doesn't simply produce drawings or paperwork; they help shape the quality of the decisions that follow. That doesn't mean spending unnecessarily, and professional fees still need to be managed carefully, but investing in the right advice at the right stage often prevents far greater costs later.

Value Engineering Continues Throughout Construction

Although the biggest opportunities usually exist before work begins, value engineering shouldn't stop there. Construction projects evolve: unexpected site conditions appear, products change, new ideas emerge. Maintaining a collaborative relationship with your contractor or construction manager allows these opportunities to be identified and assessed as the project progresses, since sometimes the best ideas don't exist when the drawings are first produced.

Better Value Isn't Always Lower Cost

One of the most common recommendations we make is actually to spend more. That may sound surprising, but every project has different priorities. A forever home will often justify investment in durability, energy performance and long-term comfort, while a development project may require spending in areas that maximise resale value.

The objective is to spend where it creates the greatest value, not simply to spend as little as possible. Not every pound invested delivers the same return.

Good Value Engineering Is Really About Good Decision-Making

The best value engineering is driven by experience, not a spreadsheet. It requires someone who understands construction methods, sequencing, procurement, programme, commercial risk and the priorities of the client.

Every project involves hundreds of decisions. The real skill lies in understanding which of those decisions matter most. Successful projects aren't simply the ones that cost the least. They're the ones where every important decision creates the greatest possible value.

02 Apr 2026

How to Compare Builders' Quotes: Why Three Prices Don't Tell the Whole Story

Most homeowners are told the same thing when they're looking for a contractor: "Get three quotes." On the surface, that sounds like sensible advice.

What happens, though, when those three quotes come back and there's £150,000 between the cheapest and the most expensive? Which one do you choose?

If the prices genuinely vary by that much, my first thought is that something has gone wrong in the process, not that one contractor is expensive and another is a bargain. The problem usually lies not in the quotes themselves, but in the process that produced them.

Comparing Quotes Starts Long Before the Quotes Arrive

One of the biggest misconceptions in construction is that comparing quotations begins once the prices land in your inbox. In reality, that's the final stage of the process. A successful tender actually begins much earlier.

At ProBuild, we think of it as three separate stages: understanding what the project should cost, issuing the right information to the right contractors, and reviewing the returned quotations properly. If any one of those stages is weak, the quotations become far less meaningful.

Step One: Understand the Budget Before Asking Anyone to Quote

This is probably the single biggest piece of advice I give clients. Before inviting contractors to price your project, you should have a realistic understanding of what the project is expected to cost. That means carrying out proper cost planning based on current market rates, not guessing.

Not only does this prevent wasting everyone's time if the available budget is significantly different from reality, but it also gives you an independent benchmark against which every quotation can later be judged. Ideally, this exercise is supported by a Bill of Quantities and, where appropriate, a Scope of Works: the Bill of Quantities breaks the project down into measurable items so everyone is pricing the same work, while the Scope of Works explains exactly what is expected to be included. Neither document guarantees identical quotations, but together they dramatically reduce misunderstanding and create a much fairer tender process.

Drawings Don't Tell the Whole Story

Many clients assume that if three contractors receive the same drawings, they'll produce broadly similar prices. Unfortunately, construction doesn't work like that. Drawings show what is being built, but they rarely explain every assumption behind how it will be built. That's where specifications, quantities and construction methodology become important.

Two experienced contractors can honestly interpret the same drawing package differently and both believe they have priced it correctly. The more assumptions each contractor has to make, the greater the differences between quotations, which is why reducing assumptions before tendering is one of the most effective ways of producing comparable prices.

You're not simply issuing drawings; you're trying to communicate everything that sits behind them.

Why Contractors Price Projects Differently

Even if every contractor is pricing exactly the same information, you should still expect differences. Every contractor operates differently. Some employ their own trades. Others subcontract almost everything. Some have long-standing supplier relationships that allow them to buy materials competitively. Others carry larger overheads because of the way their business is structured. Some include realistic allowances for future variations. Others deliberately submit a lower initial figure knowing they expect additional costs to arise later.

None of these things are immediately obvious when you compare a single number at the bottom of a quotation. That's why reviewing quotations properly requires far more than comparing prices.

Is the Cheapest Quote the Cheapest Project?

Sometimes, but not nearly as often as people assume. The cheapest quotation may represent excellent value. It may also represent missing items, unrealistic assumptions or underestimated costs that only become apparent once construction is underway. Equally, the highest quotation isn't automatically the most comprehensive.

There is no simple rule that says you should choose the cheapest, the most expensive or even the one in the middle. The objective is to establish that every contractor is competent, every quotation is pricing the same scope of work and every assumption has been properly understood. Only then can you begin making meaningful comparisons.

Look Beyond the Headline Figure

When reviewing quotations, one of the biggest warning signs is unnecessary vagueness. That doesn't necessarily mean every quotation should contain hundreds of pages of detail. Contractors are understandably cautious about providing excessive pricing breakdowns if they believe their work may simply be used to negotiate with someone else. However, a professionally prepared tender should still provide enough clarity for an experienced reviewer to identify omissions, assumptions and inconsistencies.

The objective is to understand the overall picture, not to cherry-pick individual rates. One contractor may be more expensive on concrete, another higher on plastering, and that's perfectly normal. What matters is whether the overall tender represents the same project. In other words, are you genuinely comparing apples with apples?

Choosing the Contractor Is About More Than the Price

By the time quotations are being reviewed, I would hope that every contractor invited to tender has already been assessed as capable of delivering the project. At that stage, my questions become less about price and more about delivery: who will actually be responsible for my project day to day, who will be my main point of contact throughout the build, and how many other projects will this team be running at the same time. Those answers often tell you more about how your project will be managed than the quotation itself.

What Experience Has Taught Me

During my time as Project Lead on Channel 5's Cowboy Builders, I spent nearly two years investigating residential projects that had gone badly wrong. Every project was different and every client had a different story, but one pattern appeared time and time again.

People naturally want to believe they've found a bargain, and there's nothing wrong with that. Sometimes the cheapest quotation genuinely is the best option. The problem arises when nobody has properly validated what that quotation actually includes.

Construction is rarely won or lost because of a single number; it's won or lost because of the decisions made before work even begins.

Good Tendering Leads to Good Decisions

A successful tender process isn't about collecting three prices. It's about creating the conditions for those prices to be genuinely comparable: understanding the budget before tendering, providing contractors with clear and consistent information, reviewing returned quotations professionally, and making sure you're comparing like for like before appointing anyone.

The contractor you choose will have one of the biggest influences on the success of your project. Make sure you're choosing them for the right reasons.

Ideas, Advice And Insight From The ProBuild Team.