Most single-storey extensions start with the kitchen. It’s too small, or it faces the wrong way, or it’s cut off from the garden by a wall and a narrow back door. The family lives at the back of the house, and the back of the house doesn’t work. An extension feels like the obvious answer, and often it’s a good one. Before committing, it helps to weigh up the pros and cons of a single-storey extension: what it does well, where it disappoints, and what decides the cost.
What are the advantages of a single-storey extension?
A single-storey extension is the most direct way to change how the ground floor works. It can turn a separate kitchen and dining room into one room that opens onto the garden. It keeps everything on one level, which matters with small children and for anyone planning to stay into older age. And it often avoids a full planning application, because many rear extensions fall within permitted development.
It also gives you a roof to design with. A flat roof with a large rooflight, or a pitched roof with a vaulted ceiling, can bring daylight into the middle of a deep house in a way that windows in the back wall can’t.
And you can usually stay in the house while it’s built, at least until the back wall comes out.
What are the disadvantages?
It takes garden. On a small plot the room you gain can cost you the only sunny spot outside, and that trade is best checked on a summer evening before you decide.
It can make the middle of the house darker. The old rear rooms lose their windows to the new room, so unless the extension lets daylight back in through rooflights or glazing in the roof, the hall and the middle room can end up gloomier than before.
It only adds ground floor. If the real shortage is bedrooms, a single-storey extension won’t solve it, and a loft conversion or a two-storey extension might.
And the junction between old and new is where most of the cost and most of the problems sit: the steel beam that replaces the old back wall, the drains under the new floor, and the points where the new roof meets the old wall and the new floor level meets the old one.
Does a single-storey extension need planning permission?
Often not. Permitted development lets some extensions go ahead without a planning application, provided they stay inside fixed national rules. For a single-storey rear extension in England, the main ones are:
- no more than 3 metres beyond the original rear wall of an attached house, or 4 metres on a detached one
- no more than 4 metres high, with eaves no higher than 3 metres within 2 metres of a boundary
- materials similar in appearance to the house
- the extension, with any other outbuildings and extensions, covering no more than half the land around the original house
There’s also a prior approval route that allows up to 6 metres on an attached house and 8 metres on a detached one, where the neighbours are notified and the council considers the effect on them before agreeing. Side extensions have their own rules, including being no wider than half the width of the original house.
“Original” means the house as first built, or as it stood on 1 July 1948 if it’s older, so earlier extensions count against the limits. The prior approval route and side extensions are not available under permitted development in conservation areas or the Surrey Hills National Landscape, and some homes, particularly on newer estates, have had their permitted development rights removed by a planning condition.
A lawful development certificate, the council’s written confirmation that the work is permitted, is sensible even when you’re confident, and a buyer’s solicitor will often ask for one later. Building regulations apply whichever route you take. Where planning permission is needed, a decision usually takes 8 to 12 weeks after the council validates the application. Permitted development vs planning permission sets out when each route makes sense.
How much does a single-storey extension cost in Surrey?
As a Surrey benchmark, a new extension costs £2,500–£3,500 per m² of new floor area, before VAT. Specification, site conditions and a contingency of 10–15% all move that number. The extension cost calculator lets you try your own sizes against those figures.
On jobs I tendered in 2025 and 2026, a small single-storey extension with a rewire, new heating and redecoration came back at £125,000–£135,000, excluding VAT and rounded. The extension is rarely the only work. The rest of the house usually needs something too, and it belongs in the budget from the start.
From the same tenders, a few elements give a sense of where the money goes. All are excluding VAT and rounded:
- steel to open up a rear wall: £3,000–£10,000
- a sliding or folding door 3–4 metres wide: £5,000–£6,000
- a rooflight: £3,600–£9,500
- an insulated flat roof: about £330 per m²
Is a single-storey extension worth it?
The most useful question isn’t how big you can go. It’s what isn’t working, and what is the smallest change that fixes it. Sometimes the answer is a 3-metre extension that sits within permitted development. Sometimes it’s a reworked layout with no extension at all, or a smaller one than the first builder suggested. Small extensions that make a big difference looks at where a few square metres do the most.
Once that’s clear, the size, the roof and the route through planning tend to follow.
If you’re weighing one up, the free 15-minute Clarity Call is a good first step.
Questions people ask about single-storey extensions
How far can a single-storey extension go without planning permission? Under permitted development, up to 3 metres beyond the original rear wall of an attached house, or 4 metres on a detached one, with the height and eaves limits met. The prior approval route allows up to 6 metres on an attached house and 8 metres on a detached one, after the neighbours have been notified. “Original” means the house as first built, so earlier extensions count against the limits, and some homes have had the rights removed by a planning condition.
Do I need a lawful development certificate for a permitted development extension? You are not obliged to have one, but I would get one. It is the council’s written confirmation that the work is permitted, and a buyer’s solicitor will often ask for it when you sell. Without it you are relying on your own reading of the rules. Building regulations approval is separate and applies whichever planning route the extension takes.
Can I live in the house while a single-storey extension is built? Usually, yes, at least until the back wall comes out. Most of the work happens outside the existing house to begin with. The disruptive part is the junction between old and new, when the steel goes in, the old rear wall is removed and the floors and drains are joined up. Plan for a period without a kitchen, and ask the builder how long that stage will last before you agree the programme.
