A proper extension quote itemises enabling works, drainage diversion, foundations (with a provisional sum for unforeseen depth), structural steel and design fees, roof, glazing, first and second fix, plastering, screed, decoration, external works and making good — plus statutory costs and party wall matters. Most disputes trace back to items quietly left as exclusions rather than priced.
Why extension quotes are the hardest to compare
An extension is not one job, it is a sequence of trades working over several months, and every stage has its own way of costing more than the drawing suggests. A kitchen fit-out has a fairly fixed scope. An extension has a hole in the ground whose final depth nobody knows until it is dug, drainage runs that may or may not need diverting, and a roof junction against an existing wall that only reveals its problems once the render comes off.
The result is that three builders pricing the same architect's drawing can produce three genuinely different documents: one prices only what is drawn, one prices what is drawn plus a sensible allowance for what usually goes wrong, and one prices low to win the job and expects to recover the difference through variations. All three can look similar on the total at the bottom.
Enabling works and site setup
Before any foundation is dug, someone has to clear the site, protect access, set up scaffolding, arrange skips and welfare facilities, and often take down existing structure — a rear wall, a conservatory, a chimney breast. This stage is easy to under-price because it looks like preparation rather than 'the job', but it can run to several weeks of labour and plant hire.
- Site clearance, demolition of existing structure, and disposal
- Scaffolding erection, adaptation as work progresses, and dismantling
- Skip hire and waste transfer — check how many are allowed for
- Temporary propping, weather protection, and site security/hoarding
- Protection of existing finishes, driveways and neighbouring property
Drainage diversions and services
Extending over or near an existing drain run is one of the most common sources of cost growth on a single-storey extension. If a manhole or drain falls inside the new footprint, it usually has to be diverted, which involves excavation, pipework, a new inspection chamber and, in some cases, Building Control sign-off on the diversion itself. This should appear as a distinct line, not be buried inside 'groundworks'.
- Survey of existing drain runs relative to the new footprint
- Diversion of foul or surface water drains, including new inspection chambers
- Relocation or protection of gas, water, electricity supplies and meters
- Connection of new drainage to the existing system, with fall checked
Foundations and the unforeseen-depth allowance
Foundation depth is confirmed by Building Control once trial holes are dug, based on ground conditions, and it can be significantly deeper than assumed at quote stage — particularly near trees, on clay, or close to existing foundations. A quote that states a fixed foundation price with no reference to ground conditions is making an assumption on your behalf.
The honest way to price this is a provisional sum: a stated allowance for the anticipated depth, with an agreed rate per additional course or cubic metre of concrete if the depth increases. Ask for that rate in writing before you sign, not after the hole is dug.
- Trial holes and confirmation of foundation depth with Building Control
- Provisional sum for standard foundation depth, stated explicitly
- Agreed additional rate per course of brickwork or m³ of concrete if depth increases
- Concrete type and any required reinforcement
Structural steel, design and building fabric
Where the extension involves removing a load-bearing wall, opening up a wide span, or supporting an upper floor, structural steel is usually required, and it needs a structural engineer's calculations — a distinct professional fee, separate from the architect's drawings. The quote should state whether steel is supplied and installed as a fixed price or as a provisional sum pending the engineer's specification, since sizes can only be confirmed once spans are finalised.
- Structural engineer's calculations for steel beams, padstones and lintels
- Supply, delivery and installation (including crane or hand-ball where access is tight) of steelwork
- Fireproofing/encasing of steel where required
- External wall build-up: blockwork, insulation, cavity, brick or render to match existing
- Roof structure, insulation, membrane, tiling or covering to match existing
- Windows and doors: type, glazing spec, and who supplies versus who fits
First fix, second fix and finishes
This is the stage most homeowners picture when they think 'extension', but it is also where quotes vary hugely on what is actually included. A quote can legitimately stop at 'plastered and ready for decoration' or go all the way to 'decorated, flooring laid, kitchen fitted' — both are valid scopes, but they need to be stated, because the value difference between them is substantial.
- First fix electrics, plumbing and heating (sockets, pipework, radiators/underfloor)
- Insulation, plasterboard and plastering/skimming to walls and ceilings
- Floor screed, damp-proofing and floor finish (or preparation only)
- Second fix: sockets, switches, sanitaryware, doors, skirting, architrave
- Decoration — how many coats, whether it is included at all
- External works: paths, patios, drainage gullies, making good to the garden
Professional fees and statutory costs
Alongside the building work itself, an extension carries its own set of professional and statutory costs, which are sometimes quoted separately by the homeowner's architect/designer and sometimes bundled by the builder. Either way, they should be visible somewhere in your paperwork rather than assumed.
- Architectural or design fees, and any planning application fee (see Planning Portal fee calculator)
- Building Regulations application/inspection fees, paid to Building Control or an approved inspector
- Structural engineer's fee for calculations and any site visits
- Party Wall Act matters where the extension is near a boundary or shared wall — see GOV.UK's Party Wall etc. Act 1996 guidance
- Building insurance notification during works, and any warranty/guarantee scheme fee
Party wall matters
If the extension involves work at or near a boundary, on a shared wall, or excavation within a certain distance of a neighbour's foundations, the Party Wall etc. Act 1996 may apply. This is a legal process with its own notice periods and, often, its own surveyor's fees — it is not something a builder's quote can resolve on your behalf, and a competent builder should flag it rather than stay silent. GOV.UK publishes the official explanatory guidance; a surveyor experienced in party wall matters (often found via RICS) should be consulted where it applies.
The exclusions clause — read it properly
Every extension quote should carry a clear exclusions section, and reading it tells you more than the priced items do. Common exclusions include external decoration, garden reinstatement, kitchen and bathroom supply items, curtains/blinds, any structural surprises below ground, asbestos removal if discovered, and moving/storage costs while you are without a kitchen. None of these are unreasonable exclusions in themselves — the problem is only when they are not stated, and you discover them by invoice.
Practical checklist
- Site clearance and demolition of existing structure priced as a line item
- Scaffolding hire, erection and dismantling included
- Skip hire quantity/number stated
- Drain survey and any anticipated diversion priced or flagged as provisional
- Utility relocation (gas, water, electric meters) addressed
- Foundation depth allowance stated with an agreed rate for extra depth
- Structural engineer's calculations included or identified as a separate fee
- Steel supply and installation priced (fixed or provisional pending engineer's spec)
- External wall build-up specified (insulation, cavity, brick/render to match)
- Roof structure and covering specified, including insulation standard
- Windows and doors — supply and fit responsibility stated
- First fix electrics, plumbing and heating included
- Plastering and floor screed included
- Second fix and sanitaryware included
- Decoration included or explicitly excluded
- External works (paths, patio, garden making good) addressed
- Planning and Building Regulations fees identified
- Party Wall Act applicability addressed, even if the answer is 'not applicable'
- Structural warranty or guarantee scheme mentioned where relevant
- A dated exclusions section is present and has been read in full
- Payment stages tied to visible progress, not calendar dates
Good and bad examples
What good looks like
"Foundations priced to standard trench-fill depth of 1m as a provisional sum of £X; if Building Control requires greater depth, additional depth charged at £Y per course of blockwork, agreed in writing before work proceeds. Drain survey to be carried out prior to start; any required diversion priced separately once identified." This tells you exactly what assumption has been made and what happens if it is wrong.
What to push back on
"Extension as per drawings — £48,500." A single lump sum with no breakdown gives no way to see what has been assumed about foundation depth, drainage, or where the finishes stop, and no basis for judging a variation instruction later.
Worked example
Illustrative structure of a single-storey extension quote, shown as proportions of total build cost — not market rates.
| Stage | Typical share of total | Notes |
|---|---|---|
| Enabling works & site setup | 5–8% | Scaffolding, skips, demolition, protection |
| Groundworks & drainage | 10–15% | Higher if diversion required |
| Foundations & substructure | 8–12% | Provisional sum for depth |
| Structure: walls, steel, roof | 25–30% | Includes engineer's fee |
| Windows & doors | 6–10% | Depends on glazing spec |
| First fix (electrics/plumbing) | 8–10% | |
| Plastering, screed, second fix | 10–14% | |
| Decoration & external works | 6–10% | Often partly excluded |
| Professional/statutory fees | 5–8% | Design, Building Control, party wall |
Commonly missed items
- Drain diversion left unpriced because it 'probably won't be needed'
- Foundation depth assumed rather than stated as a provisional sum
- Structural engineer's fee omitted or assumed to be covered by the architect
- External decoration and garden reinstatement excluded without saying so
- No allowance for matching existing brick/render, which can cost more than new-build finish
- Meter relocation and utility connection charges left out entirely
- Party Wall Act requirements not mentioned at all
Warning signs
- A single total figure with no stage breakdown whatsoever
- No mention of foundation depth or ground conditions anywhere in the document
- Structural steel priced as a fixed sum before any engineer's calculation exists
- No exclusions section at all — everything implied to be included
- Reluctance to put a provisional-sum rate in writing when asked directly
Questions to ask
- Is the foundation price fixed or provisional, and what happens if the depth increases?
- Has a drain survey been done, and is a diversion anticipated?
- Who is responsible for the structural engineer's fee — you or the builder?
- Is decoration included, and to how many coats?
- What is explicitly excluded from this price?
- Does the extension trigger the Party Wall Act, and who arranges any notice?
- What is the agreed rate for extra depth of foundation or unforeseen ground conditions?
When to seek professional advice
- If the quote gives no indication of how foundation depth or ground conditions will be handled
- If you are unsure whether the Party Wall Act applies to your boundary situation — consult a surveyor via RICS or read the GOV.UK guidance
- If structural steel is priced as fixed before any engineer's calculations exist
- If two quotes for the same drawing differ by more than around 15–20% with no obvious scope difference
Official sources
Frequently asked questions
- Should a builder's quote include the structural engineer's fee?
- It can go either way — some builders manage it and fold it into their price, others expect the homeowner to instruct the engineer directly. What matters is that the quote states which arrangement applies, since it affects both cost and who holds the professional responsibility.
- Why is my foundation price only a provisional sum?
- Final foundation depth is confirmed by Building Control once trial holes are dug on site, because it depends on ground conditions that can't be fully known from a drawing. A provisional sum with an agreed rate for extra depth protects both parties from guessing.
- Does every extension need Party Wall Act notice?
- Not every extension, but many do — particularly where work is at or very close to a boundary, involves a shared wall, or includes excavation near a neighbour's foundations. GOV.UK's guidance sets out when it applies; it is worth checking early since notice periods can affect your start date.
- What is normally excluded from an extension quote?
- Common exclusions include external decoration, garden reinstatement, kitchen/bathroom supply items, curtains and blinds, and any costs arising from unforeseen ground conditions or hidden defects discovered once work starts. These are reasonable exclusions provided they are stated.
- How do I compare two extension quotes with different exclusions?
- List every line item from both quotes side by side, including what each excludes, and price the gaps yourself where you can — for example, get a decorator's estimate for the excluded finishing work. The lower headline figure is meaningless until the scopes match.
Builder Proof UK publishes general educational information for homeowners. It is not legal, structural, surveying, planning, electrical, gas, insurance or financial advice, and it does not replace advice from a suitably qualified professional for your specific project.