Ideas, Advice And Insight From The ProBuild Team.

"How much will it cost?" is usually the first question anyone asks before they've even settled on a design, and it's also the hardest one to answer honestly in a single sentence. Extension costs vary enormously depending on scale, specification, location and site conditions, but it is possible to give a realistic starting point and explain what actually drives the number up or down.

As a rough guide, a straightforward single-storey rear extension in the South of England currently costs somewhere between £2,000 and £2,800 per square metre for a good standard specification. A more ambitious wrap-around extension, or one involving structural glazing, steel frames or a change in roof form, can push that to £3,000-£3,800 per square metre. Double-storey extensions are more cost-efficient per square metre than single-storey ones, because you're spreading the cost of the foundations, roof and groundworks across twice the floor area. These figures are a starting point for budgeting conversations, not a quote — every project needs a proper cost plan based on its own drawings and site.

Groundworks and foundations are often the biggest surprise. Ground conditions, existing drainage, tree proximity, and party wall considerations all affect what's needed below ground, and that work is largely invisible once complete — which makes it easy for clients to underestimate. Structural work is the next major factor: removing load-bearing walls, spanning larger glazed openings, or introducing steel beams all add cost that has nothing to do with finishes.

Specification level then does the rest. Kitchen units, bi-fold or sliding doors, roof lanterns, underfloor heating and bathroom fittings can vary by a factor of three or four between a builder-grade and a genuinely high-end specification, and this is usually where budgets creep the most because it's the part clients enjoy choosing.

Beyond the build itself, there's a list of costs that regularly get left out of early budgets: planning application and building control fees, structural engineer and architect fees, party wall agreement costs if you have neighbours either side, VAT, and a contingency sum for the unexpected — we'd always recommend at least 10% on a refurbishment or extension project, since existing buildings have a habit of revealing surprises once work starts.

The best way to avoid a mismatch between ambition and budget is to involve a principal contractor or quantity surveyor at the design stage, not after the drawings are finished. A contractor who understands build costs can flag where a design decision is going to be expensive before you've paid an architect to develop it further, and can help you make informed trade-offs — for example, choosing a slightly simpler roof form to free up budget for the glazing you actually want.

It's also worth getting quotes from contractors who price transparently, with a clear breakdown by trade rather than a single lump sum. That way, if costs do need to come down, you can see exactly where the flexibility is rather than guessing.

If you want a single rule of thumb: budget on the higher end of any range you're given until you have a proper cost plan in hand, because it's far less stressful to be pleasantly surprised than to be caught short three months into a build. A good contractor would rather have that conversation honestly at the outset than let you find out the hard way.

If you're at the early stages of planning an extension and want a realistic sense of cost before you commit to detailed drawings, we're happy to have that conversation — get in touch and we can talk through your project.

Value engineering has an unfortunate reputation. Mention it to a client midway through a project and the assumption is often that something is about to be cut or downgraded. Done properly, it's the opposite — value engineering is about protecting the client's budget and the architect's design intent at the same time, not sacrificing one for the other.

At its simplest, value engineering is the process of examining every element of a design and asking whether there's a way to deliver the same function, performance or appearance for less cost, less risk, or less programme time — without compromising what the design is trying to achieve. It's a structured, collaborative review, not an excuse to substitute cheaper materials without discussion.

The key distinction is between value engineering and cost cutting. Cost cutting removes things: a rooflight disappears, a spec gets downgraded, a room gets smaller. Value engineering finds a smarter way to deliver the same rooflight, the same spec, the same room — often by changing how something is built rather than what it looks like.

Value engineering is most effective early — ideally during design development, before drawings are finalised and certainly before tender. A structural change suggested at concept stage might take an afternoon of engineering time to explore. The same change requested after working drawings are complete, or worse, after a contract is signed, can mean abortive fees, delay, and a much smaller pool of realistic options. The earlier a contractor is involved in reviewing buildability and cost, the more genuine value engineering opportunities exist — which is one of the strongest arguments for involving a principal contractor at pre-construction stage rather than only at tender.

In residential projects, common value engineering opportunities include: rationalising structural grids and spans to use standard, readily available steel sizes rather than bespoke fabrication; reviewing glazing specifications to find products that meet the same thermal and acoustic performance at a lower cost; simplifying roof geometry where complexity is adding cost without adding anything the client will actually notice; and looking at service routes and risers early enough to avoid expensive retrofitting later. None of these require telling a client they can't have something — they're about finding a different, usually simpler, route to the same result.

Good value engineering happens in conversation with the architect and the client, not as something imposed on a design after the fact. A contractor who understands buildability can flag options the architect may not have priced, but the decision about whether a particular trade-off is acceptable should always sit with the people who understand the design intent — usually the architect and the client together. Our approach is to bring options to the table with the cost and design implications clearly explained, so decisions are informed rather than reactive.

If value engineering is done properly, the finished building shouldn't look any different to what was originally envisaged — the difference should only be visible in the budget. That's the real measure of whether it's been done well: nothing the client notices has been lost, but the numbers work.

If you're at pre-construction stage and want a contractor involved early enough to make genuine value engineering possible, rather than reactive cost-cutting later, get in touch — early involvement is where the real value gets found.