Single Storey Extensions

Often not, but it depends on your address and you should not assume either way.

Many single storey rear extensions fall under permitted development. As a general guide that allows around three metres of depth from the original rear wall on a terraced or semi-detached house and around four metres on a detached house, with limits also applying to height, proximity to the boundary and how much of the garden you cover. Deeper rear extensions can still go ahead under the prior approval process, where the council notifies your neighbours and only refuses on specific grounds.

Where it gets more complicated is the number of properties round here that sit outside those rights entirely. Conservation areas and Article 4 directions across Windsor, Ascot, Eton, Henley and Marlow remove permitted development, and a listed building needs listed building consent on top of anything else. Previous extensions to the property also count against your allowance, which catches people out.

We confirm the position for your postcode before drawing anything, whether that is West Berkshire Council, Royal Borough of Windsor and Maidenhead, Wokingham or Basingstoke and Deane. Building regulations approval is separate and applies either way.

On reasonable ground, strip footings around 900mm to 1m deep are typical. The exact depth comes from what the building inspector accepts for your soil and site rather than a rule of thumb. Two things push it deeper. Clay moves as it dries and swells, so shrinkable clay needs more depth. Trees matter more, because roots draw moisture out of clay and their zone of influence reaches well past the canopy. A mature oak near the build line can take footings past 2m or push you onto a piled or reinforced raft solution. We check this at survey rather than pricing blind.

As a rough benchmark, single storey extensions in the UK run somewhere between £1,800 and £3,000 per square metre, and the South East sits at the upper end of that because labour and materials cost more here. Treat it as a starting point rather than a quote.

One thing worth knowing is that small extensions cost more per square metre than large ones. Foundations, site setup, muck-away and professional fees barely change whether you build three metres deep or six, so those fixed costs are spread over less floor area. Going slightly bigger is often better value per square metre than going smaller.

What moves your number most is foundation depth once soil and trees are accounted for, whether structural steel is needed to open the back wall, the roof system you choose, how much glazing goes in, and access for machinery. Two extensions of identical size on the same street can land a long way apart on all of that.

Our survey is free and the written quote that follows is itemised, so you can see what each element costs and adjust the specification rather than the size.

A few things sit outside the build price and catch people out.

VAT at 20% applies to extension work, so always check whether a quote includes it. Ours do. If planning permission is needed there is an application fee, and building regulations approval carries its own charge that varies with the size of the extension.

If the work affects a shared or party wall, which is common on semi-detached and terraced houses, the Party Wall etc. Act 1996 may apply and that can mean surveyor fees depending on how your neighbours respond.

Then there is the disruption. If the kitchen is coming out you may need temporary cooking arrangements, and there is usually somewhere to store furniture for a few weeks. Worth setting aside a contingency of around five per cent of the build cost for the things nobody can foresee.

Shell and groundworks usually run three to five weeks depending on size and access, with fit-out after. A straightforward rear extension is generally finished inside eight to twelve weeks on site. The part people underestimate is what happens first, since planning and building regulations approval both take time. We give you a realistic sequence at survey rather than a best case.

Usually, and along the M4 corridor the numbers tend to work well because floor space is expensive here. Adding usable ground floor area to a family house in Newbury, Wokingham or Maidenhead is one of the more reliable improvements you can make. What matters is that the finish matches the house and the layout still makes sense afterwards. An extension that swallows the garden or leaves an existing room with no daylight can work against you, and we flag that at design stage.

If your extension goes ahead under permitted development rather than full planning permission, it is worth getting one. It is a document from the council confirming the work complied with permitted development rules, and the fee is normally half a householder planning application.

The reason to bother is resale. When you sell, the buyer's solicitor will ask for evidence that alterations were carried out lawfully, and a certificate answers that in one document instead of a scramble through old paperwork. We will tell you at survey whether your project is a candidate for one.

Under permitted development a single storey extension cannot exceed 4m in height overall, and the eaves cannot exceed 3m if the extension sits within 2m of a boundary. The ridge also cannot be higher than the ridge of the existing house. Those are the permitted development limits rather than absolute limits, so a taller design is possible with full planning permission if the council will accept it. We work the numbers for your specific property at survey.