How to Price a House Extension Before You Pay for Drawings
You should know whether you can afford a house extension before anyone draws a single line. For most homeowners, the cost is the last thing they find out.
The usual order is drawings, then planning permission, then builders' quotes. When the quotes come back above the budget, you may already have spent thousands of pounds on a design you can't afford to build.
I've built an online Estimator Engine that prices your extension before anything is drawn. In this post I go through what decides the cost of an extension, and how to work out a realistic figure for your own house.
One extension, two very different prices
No two builders price a job the same, so an honest estimate has to be a range. I see it every time I send drawings out to tender.
On a listed house knock-through I worked on recently, two builders priced exactly the same drawings. One came back at about £57,000 + VAT and the other at about £69,000 + VAT.
Most of the £12,000 gap was in the preliminaries: the cost of running the site, like skips, insurance and supervision, which never ends up in the building.
A single figure for an extension is a guess, however confident it looks. So every result from the Estimator Engine is a range, for every part of the build.
The parts of the job decide the price
A cost per square metre is the figure most people reach for, and it hides almost everything that moves the price. A better way is to price each part of the job separately: foundations, walls, roof, windows and doors, heating, finishes and the garden.
Then come the things about your house and garden that a square metre rate ignores.
How a builder gets to the back garden. With no path down the side, every barrow of rubble and every bag of cement goes through the house or along a back lane. That takes longer, and on a building site, time is money.
Room for a skip. With no driveway or front garden, you need a smaller skip, more trips, or a permit to put one on the street.
The age of the house. Any house built before 2000 needs an asbestos survey, so the builder knows what's there before cutting into the existing house.
Your own choices. Aluminium sliding doors instead of uPVC, a pitched roof instead of a flat one, stone paving instead of grass seed. Each one adds to the cost, and each one is your decision.
You might expect a terraced house to cost more to extend. On its own, it doesn't. A semi-detached or end terrace house with no side path is priced the same as a mid terrace. What costs more is the access.
Where you live changes the price
The same extension costs more in some parts of the UK than others. Materials are a small part of that. National builders' merchants sell at much the same price across mainland UK, though delivery costs more to Northern Ireland, the Highlands and the islands.
Builders' wages explain some of the difference, but far less than you'd expect. The rest comes from site access, how long the job takes, overheads and local demand.
So the Estimator Engine prices every extension from my own Edinburgh projects, then applies one adjustment to the whole estimate for where the house is. It covers England, Scotland, Wales and Northern Ireland, including the islands.
Pricing your own extension
The Estimator Engine asks plain-English questions about your house, your garden, the ground and the extension you have in mind. You answer most of them by tapping a line drawing.
Every rate comes from my own completed projects: priced tenders, invoices and final accounts, plus published material prices. I tested it against invoiced figures from real jobs, and they landed inside its ranges.
It's a financial tool, not a design tool. It answers one question: can I afford what I have in mind? You can go back and forth, changing the size, the roof, the windows or the finishes, and watch the estimate move until it fits your budget.
Before you start, read the free Before You Start guide and take its one-page worksheet out into the garden. It takes about fifteen minutes, ten of them with a tape measure at least 8m long.
The free version shows you a range for the cheapest extension for your own house, in a few minutes.
The Homeowner version, £40 at launch, emails you a full report as a PDF. It gives a range for every part of the build, a contingency, a build time, and a section called Where you are spending extra, which shows how much each of your choices adds compared with the cheapest option.
What it won't price
The Estimator Engine prices the extension itself, from about 2m by 2m up to 100 square metres. It doesn't include changes inside the existing house, kitchen units, worktops or appliances, or design, engineering and approval fees.
It gives no advice on planning permission or building regulations, and it will price an extension you would never be allowed to build. Don't use the report for a mortgage, a loan or as the basis of a contract with a builder. Everything it can't price is listed in the free guide, so you can check before you buy.
Once the cost looks affordable, talk to an architect before you spend money on design. I run 30-minute online consultations where we go through your estimate together, and I can show you options you may not have considered.
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