Single storey extensions

Single storey extensions are all unique, which means there’s no one-size-fits-all timeline. But on average, any type of home extension in the UK takes between 7 and 12 months from start to finish. That estimate includes the time spent working with an architect on design, getting planning permission if you need to, having detailed technical drawings done, selecting your contractor, plus the build itself.

Only you can decide if a single storey extension is worth it. Extending your home in any way is expensive nowadays, with the cost of labour and materials rising. But the value it can bring, both in terms of space and in your home’s valuation, outweighs the initial big spend on it for some people.

The 50% rule for a house extension is something councils sometimes consider during a planning application to make sure land doesn’t become overdeveloped. It’s not firm policy, but is a consideration they’ll often make.

The rule itself is fairly simple; you’re not allowed to extend your house more than 50% of its original footprint. If you’re not sure what the original footprint was, then it’s the size it was in 1948 (or when it was first built, if that was later). Any extensions done after 1948 (or after the date your house was originally built) are included in the 50% rule.

There’s an added complication to this rule, though. An extension also can’t take up more than 50% of he ‘amenity space’ (the garden) on your plot. So you’ll need to balance that limitation with the other 50% rule to give you the best chance of getting planning permission. By working with an architect, you can make sure your designs fall within these constraints.

Any works that affect the boundary wall, known as a ‘party wall’ will need to have a party wall agreement. To get one, you’ll need to tell your neighbours about the extension at least two months before any works start. If your neighbours give written consent to your building works, you won’t need a party wall agreement. But if they don’t reply within 14 days, or they write back in disagreement with your extension, you’ll need to have a party wall agreement commissioned. This must be done by a party wall surveyor, and the Resi team can introduce you to a trusted one if needed.

The boundary refers to the point that separates one property from another, and it’s not always a physical structure like a wall or fence. In principle, whether you have permitted development rights or planning permission, you can build right up to your neighbour’s boundary, unless it impedes their light or their privacy. It’s important to understand the rules around your boundary line if you’re doing a single storey extension, so working with an architect is a good idea to make sure your designs are compliant.

There are lots of variables in the cost of a single storey extension. But according to Resi’s data, in 2025 it costs somewhere between £50,000 and £90,000 for a basic single-story extension measuring 3m x 4m. For a larger single storey extension, around 4m x 6m, it would cost between £80,000 and £140,000. If you live in London or the surrounding areas, the cost will most likely be at the upper end of that range.

There are lots of things that can impact how much value an extension can add to your home. Things like where you live, how big a space you’ve added, and what the new rooms are used for will all be factors. As a general estimate, though, adding a single-storey extension could increase your property’s value by around 10-15%. For a better idea of how much value you could add to your house, try Resi’s free extension valuation calculator.