Quoting·8 min read

How to Quote for a House Extension

Extensions are one of the most common jobs for UK builders, and also one of the easiest to underquote. The problem isn't usually the big-ticket items — it's the prelims, the contingency, and the small tasks that eat into profit. This guide walks through how to build an extension quote from scratch.

Cost per square metre: what to charge

The most common question customers ask is 'how much per square metre?' Here are realistic 2024/25 figures for England and Wales. These are total build costs including labour and materials, at mid-spec finish.

Type	Cost range	Notes
Single storey rear extension	£1,400 – £2,200 / m²	Most common. Varies heavily by groundworks.
Two storey extension	£1,800 – £2,800 / m²	Higher per m² due to structural complexity.
Side return extension	£1,600 – £2,500 / m²	Often restricted access — add 10–15%.
Wrap-around extension	£2,000 – £3,500 / m²	Complex junction with existing structure.

Breaking down the work phases

An extension quote should follow the build sequence. This stops you forgetting items and makes it easier for customers to understand what they're paying for.

Preliminaries — scaffold, skips, temporary protection, site setup. Often 5–8% of the total.
Groundworks — strip foundations, concrete, drainage diversions. This is where estimates go wrong most often. Always visit the site before quoting if you can.
Structure — blockwork, brickwork, lintels, beam and block or suspended timber floor.
Roof structure and covering — rafters or engineered joists, felt, battens, tiles or flat roof if applicable.
Insulation and weatherproofing — cavity insulation, breather membrane, making the shell watertight.
Windows and external doors — supply and fit. Get a quote from your supplier before pricing these.
First fix — internal stud walls, electrics rough-in, any plumbing rough-in for kitchen or WC extensions.
Plasterboard and plastering — boarding, taping or skim plastering.
Second fix — skirting, architrave, door furniture, sockets and switches, fixtures.
Decoration — if in scope. Confirm with the customer — many prefer to do this themselves.
External works — making good the garden, repointing where disturbed, cleaning down.

Labour rates to use

These are realistic day rates for the UK outside London. Add roughly 15–25% for London and the South East.

Trade	Day rate
Site manager / main contractor	£280 – £400 / day
Bricklayer	£200 – £280 / day
Carpenter / joiner	£200 – £280 / day
Electrician (Part P qualified)	£250 – £350 / day
Plumber	£200 – £320 / day
Plasterer	£200 – £280 / day
General labourer	£150 – £200 / day

Materials: what to include and how to mark up

For a single-storey extension, materials typically account for 40–55% of the total build cost. Always add 10% wastage on bricks, blocks, tiles, and timber. Mark up materials by 10–20% to cover your sourcing time, delivery coordination, and the fact that you're effectively the warranty holder if something's wrong.

What builders commonly forget to include

Scaffold — a proper scaffold for a single-storey rear extension is typically £800–£1,500 for hire plus erect and dismantle.
Skip hire — budget £300–£600 per skip, and extensions usually need 2–4.
Building regulations — if you're not the one submitting, the customer pays, but make sure it's clear in writing who does what.
Structural engineer — if steel beams are involved, an SE fee of £400–£800 is common.
Temporary works — propping the existing structure when forming openings. Often missed entirely.
Party wall surveyor — required if the extension is within 3 metres of a neighbour's boundary. Customer cost, but delays your start date if not done in advance.
Contingency — add 10–15% on any extension quote. Groundworks and opening up walls always carry unknowns.

VAT

Extensions on existing homes are standard-rated at 20% VAT if you're VAT registered. New builds and some conversions are zero-rated, but extensions are not. If you're not VAT registered, make that clear in your quote — customers are often surprised when they get a quote from a VAT-registered contractor later.

Tip

When you're on site doing the initial survey, record a voice note describing every aspect of the job while you can see it. That note becomes the basis for your quote — so you're not trying to remember a job you looked at three days ago.

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More guides

How to Price Labour and Materials as a UK Tradesperson→How to Create a Building Quote by Voice→How to Win More Quotes as a Tradesperson→