How to Quote a Home Extension UK: What Builders Need to Know

Extensions are the most complex residential quoting job most builders will take on. The scope covers multiple trades, the programme runs for months, and a poorly written quote can leave you significantly out of pocket when variations start stacking up.

This guide covers what to include, how to price accurately, and how to structure a quote that protects you without putting the client off.

Before You Quote: What You Need From the Client

Don't quote an extension without:

Planning permission (if required): Is it permitted development, or has planning been granted? If neither, your quote should be conditional on planning approval.
Structural drawings / SE calculations: For anything involving RSJs, load-bearing walls, or foundations, you need structural engineer calculations before you can price accurately. Include these as exclusions or state they're subject to SE drawings.
Party wall agreement: If the extension is within 3m of a neighbour's property or shares a wall, a party wall award may be required. This is the client's responsibility to arrange, but flag it.

If you quote without these and the job requires them later, you're either absorbing the cost or having a difficult conversation.

Extension Quote: What to Include

Groundworks & Foundations

Excavate for [strip / raft / pad] foundations
Pour and cure concrete foundations to SE specification
Subfloor construction: [beam and block / suspended timber / solid concrete — specify]
DPC and DPM installation

Superstructure (Walls)

Brick / block cavity wall construction [specify block type, brick match where required]
Insulation to cavity [specify product and U-value]
Lintels over all openings

Roof Structure

[Flat / pitched / mono-pitch] roof construction
Rafters, ridge, and roof structure
Roof covering: [flat roof — EPDM/GRP; pitched — tiles to match or specify]
Insulation to meet current Part L requirements
Fascias, soffits, guttering

Openings

Install new [bi-fold / sliding / French / single] doors: [manufacturer, size — specify]
Install new windows: [specify]
Roof lantern / skylight: [specify, if applicable]

Internal Works

First fix carpentry (stud partitions if any, joinery)
First fix plumbing (extend heating, add radiators, reroute pipes)
First fix electrical (circuits, sockets, lighting positions)
Insulation to walls and floor to Part L
Plasterboard and skim
Screed floor to receive floor finish [if applicable]

Connection to Existing Building

Break through existing wall for new opening [state size]
Install RSJ to SE specification [note: SE may require specific prop/temp support]
Make good brickwork to opening
Redecorate disturbed internal areas [or exclude]

Extension Pricing Guide (UK, 2025)

These are labour + materials build cost benchmarks. They don't include professional fees (architect, SE, planning), VAT, or site-specific complications.

Extension Type	Cost Range per m²
Single-storey (basic spec)	£1,500–£2,000/m²
Single-storey (mid spec)	£2,000–£2,800/m²
Single-storey (high spec)	£2,800–£3,500+/m²
Double-storey (adds ~30–40% over single)	£1,800–£2,800/m²
Side return	£2,000–£3,000/m²
Over-garage	£1,500–£2,500/m²

Typical example: 4m x 5m (20m²) single-storey rear extension, mid spec:

Build cost: £40,000–£56,000
Architect / planning: £3,000–£8,000
Structural engineer: £1,000–£2,500
Total project: £44,000–£66,500

These figures move significantly based on London/SE England premium, specification level, and site conditions.

Extension Quote Template (Summary Structure)

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[COMPANY NAME]
[Address] | [Phone] | [Email] | [VAT No.]
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EXTENSION BUILD QUOTATION

Ref: [REF]  |  Date: [DATE]  |  Valid: 30 days
For: [Client Name], [Property Address]

PROJECT: [e.g. Single-storey rear extension — 
4m x 5m — kitchen diner, flat roof with lantern, 
bi-fold doors to garden]
Based on: Planning permission [ref], SE drawings 
[ref / to be provided], architect spec [ref]

━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

SCOPE [detailed phase by phase as above]

COST BREAKDOWN
Groundworks & foundations:        £[X]
Superstructure (walls/roof):      £[X]
Openings (doors/windows):         £[X]
First fix (plumbing/electrical):  £[X]
Insulation & airtightness:        £[X]
Plastering/screeding:             £[X]
Internal connection to house:     £[X]
Scaffolding:                      £[X]
Waste disposal:                   £[X]
Contingency [optional — 5–10%]:   £[X]
                                ──────
Sub-total:                        £[X]
VAT @ 20%:                        £[X]
                                ──────
TOTAL:                            £[X]

EXCLUSIONS
• Architect / planning / SE fees
• Party wall surveyor costs
• Building control application fees
• Kitchen / bathroom fit-out beyond shell
• Floor finishes, kitchen units, sanitaryware
• External landscaping / making good garden
• Decorating throughout

PROGRAMME
Start: [Date — subject to planning / SE drawings]
Duration: [X] weeks
Note: programme assumes [weather / material 
delivery / structural opening timing] conditions

PAYMENT SCHEDULE (STAGE PAYMENTS)
Deposit (15%) on acceptance:          £[X]
Stage 1 — Foundations complete:       £[X]
Stage 2 — Wall plate level:           £[X]
Stage 3 — Watertight:                 £[X]
Stage 4 — First fix complete:         £[X]
Stage 5 — Plastered out:              £[X]
Final — Practical completion:         £[X]
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The 10% Contingency Debate

Some builders include a contingency (usually 5–10%) explicitly in their extension quotes. Others absorb it silently.

The argument for showing it: transparency. The client knows what they're paying for and why. It signals honesty.

The argument against: some clients see it as padding and try to negotiate it out.

Our view: include it as a line item, and explain it on the site visit: "Extensions always have surprises — groundwater, unexpected foundation conditions, services in the way. This is your buffer so neither of us is caught out. If we don't need it, we'll agree what to do with it at the end."

Most clients who understand what an extension involves will appreciate this.

Making Extension Quotes Visual

Extensions are high-stakes projects. Clients are spending £30,000–£80,000+ on their home, and the majority of that commitment happens at the quote stage, before a single brick is laid.

A visual proposal — showing before and after, what the new space will look like from the garden, how it connects to the existing house — is one of the most powerful tools available to a builder at this stage. It reduces anxiety, builds excitement, and positions you as someone who understands the vision as well as the build.

Stop Losing Jobs on Paperwork

Speqly builds visual proposals from your site photos — before/after AI visualisations, itemised costs, stage payment schedule, and a one-tap accept button.

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