Building an extension above a garage
Two things decide whether you can build a room over your garage. First, whether the garage was built to carry a second storey — most single garages were not. Second, planning permission, which you will almost always need, because a storey added to the side of a house is not permitted development. Get the structural answer before you spend a penny on drawings.
The short answer
An extension above a garage is one of the cheaper ways to add a proper double bedroom, because the footprint and one wall of the structure are already there. That is the appeal, and it is real. The catch is that the garage underneath usually was not designed to hold anything heavier than its own roof.
Most attached garages sit on shallow foundations and have single-skin, half-brick walls. Neither is built for the weight of a floor, a room, furniture and a new roof on top. So the first person you call is not an architect — it is a structural engineer, who digs a trial pit against the footings and tells you whether you are building on what is there, strengthening it, or knocking the garage down and starting again. That single finding shapes your budget more than any other decision on the job.
On planning, assume you need an application. A full storey added above a garage that sits to the side of your house counts as a two-storey side extension, and those are never permitted development.
Will your garage take another storey?
This is the question the whole project turns on, and you can get a long way towards the answer before you commission anything. Walk out and look at the garage walls. If they are a single thickness of brick — roughly 100mm, one brick’s width — they are almost certainly not strong enough for a room above without strengthening or rebuilding. A cavity wall, two skins with a gap between, stands a better chance, though it still needs checking.
Foundations are the bigger unknown, because you cannot see them. Garages built in the 1960s and 70s often sit on a shallow slab or a narrow strip of concrete that was never meant to spread the load of a second storey. A structural engineer will open a trial pit against the footing, measure its depth and width, check the soil, and calculate whether it can carry the extra load. Expect to pay from a few hundred pounds up to around £1,400 for that inspection and the calculations — money that can save you tens of thousands by catching an unbuildable garage early.
If the foundations fall short, underpinning is the usual fix: excavating beneath the existing footing in short sections and pouring new concrete down to a proper depth. It is disruptive and not cheap — commonly £5,000 to £15,000, depending on how much of the perimeter needs it. Sometimes the engineer’s verdict is blunter: the garage is not worth saving, and a new two-storey structure on fresh foundations works out cheaper and better than propping up the old one. That is not the plan failing; it is the plan telling you what it really costs.
The planning position
Where the garage sits on your house decides how the rules read. A garage attached to the side means a storey above it is a two-storey side extension — and permitted development only ever allows single-storey side extensions. So a room over a side garage needs a full householder planning application, with no way around it.
If the garage is at the rear, a two-storey rear extension can in principle be permitted development, but the limits are tight and easy to trip over. The figures below are the ones that catch people out.
| What you are checking | The rule (England) |
|---|---|
| Two-storey side extension | Never permitted development — always needs a planning application |
| Two-storey rear extension depth | No more than 3m beyond the original rear wall |
| Distance to the rear boundary | No part within 7m of the rear boundary |
| Upper-floor side windows | Obscure-glazed and non-opening below 1.7m from the floor |
| Materials and roof | Similar to the existing house; roof pitch matched where practicable |
| Householder application fee | £548 (England, from 1 April 2026) |
Sources: Permitted development rights for householders technical guidance (GOV.UK) and the Planning Portal; the householder fee is set nationally by the Town and Country Planning (Fees) Regulations, at £548 from 1 April 2026. Designated land — conservation areas, National Parks, AONBs — removes side-extension permitted development rights entirely, and some estates have them removed by a planning condition, so confirm with your local planning authority before relying on any of this. For the wider picture, see our guide to permitted development rules.
Building Regulations and the party wall
Planning permission is only half the paperwork. Building Regulations apply to a room over a garage without exception, and here they bite harder than on an ordinary ground-floor extension. Building Control will want structural calculations for the new floor and for the strengthened or rebuilt garage below, proper fire separation between the garage and the house, insulation to current standards, and a safe means of escape from the new upstairs room. Our guide to building regulations for extensions covers what the process involves.
If your garage sits on or against the boundary with a neighbour — as attached garages often do — the Party Wall etc. Act 1996 almost certainly applies. You will need to serve written notice on the neighbour before work starts, usually two months ahead for work to a party wall. It is a legal step, not a courtesy, and skipping it can stall the job or land you in a dispute once the scaffold is up.
What it costs, and whether it is worth it
As a rough guide, building the room itself — once the structure below is sound — runs to somewhere around £1,400 to £2,000 per square metre, before you add the cost of any underpinning or a garage rebuild underneath. On top of the build come the engineer, the planning fee, Building Control and drawings. A modest double bedroom over a single garage is a smaller job than a full two-storey extension, but the structural work below can quietly close that gap.
These figures come from BestBuilders’ 2026 regional cost model (published national cost guides × a regional labour index, averaged across 519 UK towns); they are indicative, not a quote for your house. For a fuller breakdown by size and region, see BestBuilders’ garage extension cost guide.
Is it worth doing? Often, yes — a well-built bedroom, and where the layout allows an en-suite, can add real value and solve a bedroom shortage without eating into the garden. But run the maths honestly. If the engineer finds you need full underpinning or a rebuild, weigh the total against a fresh two-storey extension elsewhere on the house, or against a simpler garage conversion if a ground-floor room would do the job. The cheapest route to the space you need is not always the one that reuses the garage.
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