What Is Included in a House Extension Quote and What Isn’t?
A standard house extension quote covers groundworks and foundations, external walls, the roof structure and its covering, structural steelwork, window and door frames, first fix plumbing and electrics, and plastering. What it usually leaves out is often the bigger surprise: the kitchen itself, flooring, decoration, underfloor heating, fitted furniture, and every professional fee. Design, structural engineering, and building regulations are almost always priced and handled separately.
This guide goes through every line item in detail: what’s standard, what costs extra, and which hidden costs catch most homeowners out before they’ve thought to budget for them. Real house extension cost examples are used throughout, so every figure here reflects an actual Essex build rather than a rough estimate.
What a Standard House Extension Quote Includes
- Groundworks. Digging out the foundation trench, removing the spoil, and any tree root protection or ground prep the site needs. If ground conditions call for piled foundations rather than a standard trench, that’s usually priced as its own separate line, not folded into groundworks.
- Foundations. The concrete pour and any reinforcement. Standard strip foundations are part of the core quote; piled foundations are almost always an addition on top.
- External walls. Blockwork, brickwork, or whatever outer skin has been specified, plus cavity insulation and the inner leaf.
- Structural steelwork. The beams needed for any wall removals or large openings. A structural engineer works out exactly what’s required; the builder then supplies and fits it.
- Roof structure. The timber frame or engineered joists underneath, decking for a flat roof, rafters for a pitched one.
- Roof covering. Felt and gravel, GRP, or an EPDM membrane on a flat roof; tiles or slates on a pitched one. Glass lanterns and rooflights sometimes come with the quote and sometimes don’t; always worth confirming which.
- Windows and external door frames. The frames themselves. The glazing can sometimes be a separate line, depending on the builder.
- First fix plumbing. Waste runs, soil pipes, and any new drainage connections the extension needs.
- First fix electrics. A consumer unit upgrade if one’s needed, cable runs, and back boxes ready for sockets and switches.
- Plastering. Walls and ceilings brought to a finish that’s ready for decoration, not decorated itself.
Kirkwood’s view The item that causes the most confusion between two quotes is almost always the glazing. One builder’s price might include supplying and fitting the glass lantern; another might only cover fitting a lantern the homeowner buys separately, or leave bi-fold door hardware and locking systems as an add-on. The same pattern shows up with underfloor heating: one quote includes the manifold and pipework, another only covers the screed on top of a system someone else installs. Two quotes can look wildly different on the surface and simply be covering different things. The fix is always the same: ask exactly what’s included for glazing, heating, and hardware before comparing figures side by side. |
An Extension Quote Example — What a Good Breakdown Looks Like
A properly itemised quote breaks the build down stage by stage and trade by trade, with labour and materials shown separately for each one. That’s what makes it possible to actually compare two quotes, trim the budget by adjusting a specific item, or challenge a figure that looks off. A one-page number with nothing behind it gives you none of that; there’s nothing to compare, nothing to negotiate, and nothing to check.
Here’s an extension quote breakdown for a standard 4m x 4m single-storey rear extension in Essex in 2026, showing what each stage typically costs:
Stage | Typical Cost (Essex 2026) |
Groundworks and foundations | £6,000 – £10,000 |
External walls and superstructure | £12,000 – £18,000 |
Roof structure and covering | £5,000 – £9,000 |
Structural steelwork | £2,500 – £5,000 |
Windows, doors and glazing | £5,000 – £12,000 |
First fix plumbing and electrics | £3,500 – £5,500 |
Plastering | £2,500 – £4,000 |
External works and making good | £1,500 – £3,000 |
TOTAL SHELL BUILD | £38,500 – £66,500 |
These are shell build figures only. They cover everything in the section above: the structure, the roof, the first fix, but nothing beyond it. The next section covers exactly what’s not included, and it’s usually where the real budget surprises turn up.
What Is NOT Included — the House Extension Hidden Costs Homeowners Miss
This is the section most homeowners actually need and least expect. A standard shell build quote almost never includes the following:
Item | Typical Cost | Note |
Kitchen units and appliances | £10,000 – £40,000+ | Mid-range £10k–£25k; high-spec can exceed £40k |
Flooring | £50 – £150 per m² | Tiles, engineered wood, or LVT, supply and fit |
Decoration | £1,500 – £3,000 | Painting and decorating the finished shell |
Underfloor heating | £3,000 – £6,000 | System and manifold, even if the builder fits it |
Bi-fold/sliding door hardware | £500 – £1,500 | Frames often included; leaf hardware and locking may not be |
Garden reinstatement | £500 – £2,000 | Making good the area dug up for foundations, turf to paving |
Professional fees | 10–15% of build cost | Design, structural engineering, building regs |
VAT | +20% | Most builders quote ex-VAT |
Kitchen units and appliances are the single biggest exclusion by value; a mid-range fitted kitchen runs £10,000 to £25,000, and a high-specification one can clear £40,000.
Flooring is entirely separate, typically £50 to £150 per m² supply and fit depending on material.
Decoration — painting and finishing the new space- usually costs £1,500 to £3,000 on a standard extension.
Underfloor heating is nearly always its own line item, even when the same builder is happy to fit it, and typically runs £3,000 to £6,000.
Professional fees — design, structural engineering, and building regulations- sit outside the build cost entirely and typically add 10 to 15 per cent on top.
VAT is the one that catches people cold: most builders quote ex-VAT, so add 20 per cent to whatever figure you’re looking at before treating it as final.
Kirkwood’s view The item that catches homeowners off guard most consistently is the kitchen. People budget for the build and assume the kitchen is somehow part of that number, then discover a mid-range fitted kitchen alone can run £10,000 to £25,000 on top. The other recurring surprise is professional fees, design, structural engineering, and building regulations, landing as a separate cost late in the process, when they should really be factored in from day one. Both are avoidable with a proper conversation before the build quote is even requested, which is exactly what the Discovery Call is for. |
Not sure what your quote actually covers? Kirkwood reviews quotes with clients as part of the service. Book a free Discovery Call. |
Professional Fees — What They Cover and What They Cost
Most homeowners open a build quote assuming it covers the whole project. It never does; the professional stages that get the project to the point where a builder can even quote sit entirely outside it. There are four of them:
Stage | Typical Cost |
Architectural design (drawings, survey, planning application) | £1,300 – £1,600 |
Structural engineering (foundation design, beam calculations) | £1,000 – £1,600 |
Building regulations drawings (technical construction package) | £600 – £1,000 |
Building Control fee (council or private inspector) | £800 – £1,500 |
TOTAL PROFESSIONAL FEES | £3,700 – £5,700 |
Add professional fees to the build cost, and you get the true total for the project, not just the shell. Kirkwood provides all four stages in-house, with a fixed fee for each agreed before any work begins, so there’s nothing to discover partway through. For a fuller breakdown of total costs by extension type, see our house extension costs guide, or our cost per m² breakdown for Essex.
A house extension quote covers the shell, the structure, the roof, the first fix, but not the kitchen, the flooring, the decoration, the underfloor heating, or the professional fees that got the project approved in the first place. Knowing that before you start comparing quotes is the difference between a meaningful comparison and a confusing one. For a realistic total cost on your own project, book a free Discovery Call with Kirkwood.
Book a Free Discovery Call
No, you don’t need a registered professional carrying that specific title for a house extension. What you do need is proper planning drawings, structural engineering, and building regulations drawings, and Kirkwood produces all three in-house for extensions across Chelmsford, Brentwood, Billericay, Wickford, Basildon, Rayleigh, Leigh-on-Sea, Southend, and the surrounding areas of Essex.
Frequently Asked Questions
What does a house extension quote include?
A standard quote covers groundworks and foundations, external walls, roof structure and covering, structural steelwork, window and door frames, first fix plumbing and electrics, and plastering. It doesn’t usually include the kitchen, flooring, decoration, underfloor heating, or professional fees such as design, structural engineering, and building regulations. Always ask for an itemised breakdown to confirm exactly what’s covered.
What is not included in a house extension quote?
The items almost always left out are kitchen units and appliances (typically £10,000 to £40,000 extra), flooring (£50 to £150 per m²), decoration (£1,500 to £3,000), underfloor heating (£3,000 to £6,000), garden reinstatement, and professional fees. Professional fees alone- design, structural engineering, building regulations, and building control- typically add another 10 to 15 per cent on top of the build cost.
How much are professional fees for a house extension?
For a standard single-storey extension in Essex, expect £3,700 to £5,700 in total. That breaks down into architectural design from £1,300, structural engineering from £1,000, building regulations drawings from £600, and a building control fee from £800. These fees always sit outside the builder’s quote and need budgeting separately.
What does a shell build mean for a house extension?
A shell build is structurally complete and weathertight, plastered throughout, and has first-fix plumbing and electrics already in, but no flooring, kitchen, decoration, or second-fix fittings. It’s ready for the homeowner to fit out. Most extension quotes are for a shell build only, and the fit-out on top can add anywhere from £15,000 to £60,000 depending on specification.
What is an itemised house extension quote?
An itemised quote splits the build cost into individual stages: groundworks, foundations, external walls, steelwork, roof, glazing, first fix, plastering, and external works, with labour and materials shown separately for each. That’s what makes a real comparison between quotes possible. A single headline figure with no breakdown behind it can’t be meaningfully compared to anything.
Does a house extension quote include VAT?
Not always, and it’s worth checking every time. Most builders quote ex-VAT, excluding it, so always confirm whether a figure includes VAT before comparing it against another quote. At 20 per cent, it’s a real number: on a £50,000 quote, VAT alone adds £10,000.
Does a house extension quote include the kitchen?
No, almost never. The quote covers the shell, walls, roof, glazing frames, plumbing to first fix, electrics to first fix, but not the kitchen units, appliances, worktops, or fitting. Budget £10,000 to £25,000 for a mid-range kitchen, and £40,000 or more for a high-specification one.
How do I compare two house extension quotes?
Compare them line by line, not headline to headline. Check that both cover the same scope, the same foundation type, the same steelwork specification, the same glazing standard. The most common reason one quote comes in lower is that it’s simply left something out that the other included. If a figure looks significantly lower, ask exactly what’s been allowed for on foundations and steelwork before accepting it. Our guide on questions to ask your builder covers this in more depth.
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