If you're weighing up a rear or side extension on a Brighton terrace, Victorian villa or 1930s semi, the honest answer is that cost varies a lot more than the headline figures you'll find online. Here's what actually drives the price locally, with realistic ranges rather than guesswork.
For a straightforward single-storey rear extension in Brighton, builders typically price between £2,200 and £2,900 per square metre for a good standard of build, including a basic kitchen fit-out shell but not bespoke units or flooring. So a modest 4m x 3m extension (12 square metres) often lands somewhere between £26,000 and £35,000, while a larger 6m x 4m kitchen-diner extension (24 square metres) can run £53,000 to £70,000.
Double-storey extensions cost more in total but less per square metre, usually £1,700 to £2,300 per square metre, because you're sharing one roof and one set of foundations across two floors. A 4m x 3m double-storey addition giving you extra living space downstairs and a bedroom above might come to £45,000 to £62,000.
Wraparound extensions, popular on the side-return terraces around Hanover, Fiveways and Preston Park, tend to sit at the higher end because of the extra corner junctions, roof detailing and often a steel frame to open up the kitchen into the garden.
Ground conditions matter more in Brighton than many homeowners expect. Properties on the chalk downs around Woodingdean or Westdene generally have stable, straightforward foundations, but many of the city's Victorian terraces in Hanover, Round Hill and Kemptown were built on shallow footings that need underpinning or deeper trench foundations before an extension can be tied in, which can add several thousand pounds to the groundworks.
Access is another local factor. Narrow Victorian streets with no off-road parking mean skips, concrete deliveries and scaffolding often need a parking suspension or temporary traffic arrangement through Brighton & Hove City Council, and materials sometimes have to be carried through the house rather than lifted straight into the garden.
Structural work also drives cost. Opening a kitchen into a side-return or rear extension usually means a steel beam sized by a structural engineer, and the more of the original rear wall you remove, the bigger and more expensive that steel becomes.
Many single-storey rear extensions fall under permitted development and don't need full planning permission, provided they stay within depth, height and boundary limits. However, Brighton has an unusually large number of conservation areas, including Clifton Hill, Montpelier, Round Hill, Kemp Town and Cliftonville, where permitted development rights are often restricted and a planning application is required even for modest work. It's worth checking your address against Brighton & Hove City Council's conservation area maps before you budget.
If your property is terraced or semi-detached, which covers most of central and east Brighton, you'll almost certainly need a Party Wall Agreement with your neighbour before digging foundations close to the shared wall. Budget a few hundred pounds for a surveyor if your neighbour doesn't consent informally, and allow several weeks for the process so it doesn't delay your start date.
A householder planning application currently costs around £250, and you'll also need Building Regulations approval, either through the council or an approved inspector, which typically adds £600 to £1,200 depending on the size and complexity of the extension.
The construction price is rarely the only cost. Architectural drawings or a designer's package for a typical Brighton extension runs £1,500 to £4,000, a structural engineer for calculations and steel sizing is usually £500 to £1,200, and if you need party wall surveying, that's on top again.
Beyond the shell, kitchen units, flooring, bifold or sliding doors, and any underfloor heating are usually priced separately and can easily add £10,000 to £20,000 to a kitchen extension once you choose finishes. It's sensible to hold back a contingency of 10 to 15% of the build cost for unexpected items, which on an older Brighton property is often where you find things like outdated drainage, old coal cellars, or asbestos in a garden wall that need sorting before work continues.
Many single-storey rear extensions are covered by permitted development, but Brighton has a large number of conservation areas, such as Clifton Hill, Montpelier and Kemp Town, where permission is usually required. Always check your specific address against Brighton & Hove City Council's planning portal before assuming permitted development applies.
A single-storey rear extension on a Brighton terrace usually takes 10 to 16 weeks on site once work starts, while a double-storey extension often takes 14 to 20 weeks. Older properties with underpinning, awkward access or party wall negotiations can add several weeks to that timeline.
A well-built kitchen extension or extra bedroom often adds meaningful value in Brighton, particularly in family-friendly areas like Hanover, Fiveways and Westdene where buyers pay a premium for extra space. That said, value added depends heavily on your street, the quality of the finish, and not over-building beyond what similar local homes sell for.
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