How to Price an Extension — What Builders Need to Consider

Extensions are the bread and butter of many builders — good, sustained work with a defined scope. They're also the category where underpricing is most common and most damaging, because the consequences play out over months rather than days.

Here's how to price them so the numbers work.

The rule before anything else Never price an extension without seeing the plans. If there are no plans, tell the customer to get plans drawn up before you provide a fixed price. Pricing from a sketch on a napkin or a description over the phone is how you end up committed to a price for something that turns out to be twice the work you envisioned.

What drives the cost?

Extension cost varies enormously based on:

Size — obviously. Price per square metre is a useful benchmark once you know your own numbers. Foundation type — standard strip foundations vs piled foundations (sloping ground, poor soil, proximity to trees) is a significant cost difference Structural complexity — a simple lean-to vs an extension that requires steels, structural calculations and building control sign-off Specification — a basic brick-and-block build vs full-fill insulation, triple glazing and a heritage-style exterior Access — tight side return, restricted deliveries, need for hoarding or a skip permit Finishes — is the price to watertight shell only, or does it include first and second fix, plastering, flooring, decoration? Service diversions — gas, electric, drains that need moving

Pricing by phase For a typical extension, think in these phases:

Groundworks — excavation, foundations, drainage Structure — walls, steels, roof structure Roof covering — tiles, membrane, fascias and gutters External glazing — windows and doors Masonry and insulation First fix services — if you're managing the electrician and plumber Internal — stud walls, boarding, plastering (or manage subbies) Second fix services Finishes — flooring, decoration (or manage subbies) External works — making good the garden, path, drainage

Pricing each phase separately, even in a summary quote, helps you spot where time and money actually go — and makes it easier to present a phased payment structure to the customer.

Get fixed supplier prices before you quote

Timber prices, block prices, roof tile prices and glazing prices all move. Don't quote from memory or last year's job. Get current prices from your merchant and glazing supplier before the quote goes out — especially on a job that won't start for two or three months.

Build in a materials escalation clause for larger jobs — something that allows you to revisit the materials element if there's a significant price movement between quote and order.

Subcontractors — manage them carefully

Most builders subcontract electrical, plumbing and plastering at minimum. When you quote a price to the customer that includes these trades, you're taking the risk that your subbies' prices hold.

Get written quotes from your subbies before your quote goes out. Don't use estimates from past jobs or assumptions about what someone will charge. And build a margin into their cost — managing subbies on a job takes time that isn't free.

The contingency question For extensions, a 10-15% contingency on the labour element is reasonable. On groundworks specifically, go higher — ground conditions are genuinely uncertain until you're digging.

Put the contingency in your price, not as a separate line item. A customer who sees a "contingency" line queries it. A customer whose quoted price reflects proper risk coverage is getting an honest quote.

Payment structure

For a job running two to four months, staged payments are the only sensible approach:

Deposit on instruction: 10-15% Foundations complete: 20% Watertight shell: 25% First fix complete: 20% Practical completion: 20% Final retention on sign-off: 5%

Never be more than one stage ahead of the customer in terms of money owed. If you're two stages ahead and they stop paying, you've funded work you'll never recover.

Benchmark prices in 2026

For a mid-spec single-storey rear extension in the UK:

London and South East: £2,500–£3,500 per square metre Midlands: £1,800–£2,500 per square metre North: £1,600–£2,200 per square metre

These are total build costs including labour and materials. If you're pricing labour only, your portion is roughly 30-40% of the total.

The number that tells you if you've priced it right When the job is finished, divide your profit by the number of days you worked. That's your effective day rate. If it's below what you'd have earned doing day rate work elsewhere, you priced it wrong. Track it every time and your quoting accuracy will improve.