Rear extension quote checklist: every line a complete quote should have

A line-by-line checklist for a single storey rear extension quote in the UK, with the items builders most often leave out and the cost surprises they cause.

Use this against the quote in front of you

Tick each item off against the builder's document. Anything not mentioned is either included silently (ask them to confirm in writing) or coming later as an extra.

Preliminaries and site

• Site set-up, welfare, temporary protection to your home
• Scaffolding or edge protection
• Skips and waste, with the number allowed for
• Party wall notices where the extension is within 3 metres of a neighbour's foundations or touches a shared wall

Below ground

• Strip-out and demolition including breaking through the existing rear wall
• Excavation with a depth assumption and a rate per additional 300 mm if building control require deeper trenches
• Foundations: trench fill or strip, concrete specified
• Drainage alterations, new gullies, and a build-over agreement with the water company if a sewer runs under the footprint
• Damp proof course and damp proof membrane, cavity trays and weep vents

Structure and envelope

• Floor: hardcore, sand blinding, membrane, insulation, slab or beam and block, screed
• Walls: brick to match, block, cavity width and insulation type and thickness
• Structural steel, lintels and padstones over the new opening and the knock-through, with the engineer's calculations
• Roof: joists and spans, warm or cold construction, covering (GRP, EPDM, tiles), insulation and U-value, fascias, soffits, guttering and downpipe
• Windows and doors: manufacturer, material, glazing specification, configuration of any bifold or sliding doors

Services and finishes

• First and second fix electrics, number of sockets and lights, consumer unit capacity, Part P certificate
• Heating extended into the new room: radiators or underfloor, pipework, and whether the boiler can cope
• Plumbing for any sink or utility
• Plastering to walls and ceilings
• Decoration: mist coat and finish coats
• Skirting, architrave, internal doors
• External making good: patio levels, thresholds, paths

Fees, terms and paperwork

• Building control application and inspection fees, and who applies
• Structural engineer's fee
• Contingency allowance and how variations are priced and agreed in writing
• VAT position, and the VAT-inclusive total
• Programme with start and completion dates
• Payment schedule tied to completed stages, not a large deposit
• Public liability insurance and workmanship guarantee

What tonight's checks typically find

Curtilage checked a real eight-line extension quote of £25,400 excluding VAT and found no floor, no steels, no drainage, no heating, no building control or engineer fees and a 50 percent deposit. That is not an unusual quote. Upload yours and see what is missing before you sign.

Updated 2026-09-14. Curtilage is not legal advice.