What's Included in an Extension Quote? An Itemised Example

One of the most common ways homeowners get burned is signing off a builder's quote without understanding what it actually covers. A vague "extension: £45,000" quote almost always ends up costing more, because the things that are not specified become extras during the build. Here is what a proper, itemised quote should contain — and what to push back on if yours does not.

The 12 sections every extension quote should cover

A complete extension quote breaks the build down into clear stages. Each stage has a line for materials, labour, and subcontractor costs. The structure we use on every JCT Building quote:

1. 1.Site preparation, access and welfare
2. 2.Groundworks, excavation, drainage diversions
3. 3.Foundations (mass concrete or trench fill)
4. 4.Structural steelwork (RSJs, padstones, lintels)
5. 5.Brickwork, blockwork, cavity insulation
6. 6.Roof structure, felting, tiling, lead flashings
7. 7.Doors, windows, glazing (including bi-folds, lanterns)
8. 8.First fix electrics, plumbing, heating
9. 9.Plasterboarding, insulation, sound deadening
10. 10.Plastering, screeding, second fix electrics and plumbing
11. 11.Decorating, flooring, internal joinery
12. 12.Building Control inspections, certificates, snagging

An itemised example: 4m × 5m single-storey rear extension

Here is a real-world cost breakdown for a typical 20m² rear extension on a semi-detached property in Kidderminster, completed in 2026. Numbers are rounded and based on standard specification:

• Site preparation, skip, welfare: £1,200
• Groundworks and excavation: £3,400
• Foundations and concrete: £4,800
• Steelwork (single RSJ and padstones): £3,600
• Brickwork, blocks, insulation: £8,500
• Roof structure, tiles, lead work: £6,800
• Bi-fold doors (3.6m, standard spec): £4,200
• Roof lantern (1.5m × 1m): £2,400
• First fix electrics, plumbing, heating: £3,800
• Plasterboarding, insulation: £2,900
• Plastering and screed: £3,400
• Second fix electrics, plumbing, sockets: £2,600
• Flooring (porcelain tile + screed): £2,800
• Decorating: £1,800
• Building Control fees and certificates: £680
• Skip, snagging, final clean: £900

Total: £53,780. The actual quoted price would round to £53,500–£55,000 to include a small contingency for unknowns (typically 3–5% on a straightforward build).

Red flags to watch for

These are the patterns that usually signal trouble:

• "Provisional sum: £X,000" without a clear scope — this is the builder hedging that they have not fully priced something. Push back and get it priced properly.
• No separate line for steelwork or structural design — these are non-optional and add £3,000–£8,000. If missing, they will reappear as an extra.
• No mention of Building Control fees or certificates — these always cost £500–£1,000 and must be included.
• "Kitchen included" without specifying brand, model, or supply chain — kitchens can vary by £8,000–£30,000+ for the same footprint.
• "Standard finish" without a written spec — defines the difference between £35 plasterboard and £200 designer plaster finish per square metre.
• Single-line quotes ("extension: £45,000") — these hide everything and almost always end up costing more.

What is typically excluded from a builder's quote

Even a complete quote usually excludes certain items, which the homeowner buys separately. The quote should list these clearly so you can budget for them:

• Kitchen units, worktops, appliances (£8,000–£30,000)
• Architect's drawings and planning fees (£1,500–£5,000 + £206 council fee)
• Structural engineer's calculations (often £400–£800)
• Connection to existing decoration in adjoining rooms
• Garden landscaping after the build
• External electrics (lights, sockets, EV charge points)

Payment schedule

A good quote also includes a payment schedule tied to milestones — not arbitrary calendar dates. A typical structure on a 12-week project:

1. 1.Deposit on sign-off (10–15%): confirms booking, orders materials
2. 2.On completion of foundations (15%): typically week 2
3. 3.On weather-tight stage (25%): roof on, windows in, typically week 5
4. 4.On first-fix completion (20%): electrics, plumbing in, walls plastered, typically week 8
5. 5.On second-fix completion (15%): kitchen fitted, finishes installed, typically week 10
6. 6.Final payment on snagging completion (10–15%): typically week 12

Avoid builders who ask for 50% upfront — that is a serious red flag. Reasonable deposits are 10–20%.