Most people start with the wrong question: how much per square metre. That number helps, but on a typical South London house the cost is often set by things you cannot see, such as clay soil, a sewer under the garden, a shared wall with next door or a skip with nowhere to go. This guide gives honest ranges and explains what pushes a quote up or down, so you can budget before you call anyone.
These are broad figures for 2026 and cover the build shell, first fix and a standard finish. Your actual cost depends on the house, the design and the specification you choose. Kitchens, flooring and fitted furniture are usually priced separately.
Per square metre, many South London projects come in at roughly £2,500 to £3,500 for a single storey extension at a mid-range finish. Large areas of glazing, steel frames or a high-end finish can push that past £4,000. A double storey extension often costs a little less per square metre because the foundations and roof serve two floors. The total is still much higher.
Much of South London sits on London clay, which shrinks and swells with the weather. Near mature trees, which are common along streets in Croydon, Norwood and Streatham, building control may ask for deeper foundations, heave protection or even piling. That can add several thousand pounds, and sometimes much more, before anything shows above ground.
Terraced streets bring their own costs. Many houses have no side access, so every bag of cement and every barrow of spoil goes through the house. Skips and grab lorries need a council permit, and on busy roads you may need a parking suspension as well. Many Victorian terraces also have a shared public sewer running under the back garden. Building over or near it needs a build-over agreement from Thames Water, and sometimes a redesign of the foundations.
The build quote is not the whole bill. On a typical project, fees and approvals can add 10 to 20 percent on top.
Many single storey rear extensions can go ahead under permitted development: up to 3 metres deep on a terrace or semi, or 4 metres on a detached house. The prior approval route can extend that to 6 or 8 metres. Keeping within these limits can save months and some fees.
Things change if you live in a conservation area or somewhere covered by an Article 4 direction. There are many across Croydon, Lambeth, Southwark and Bromley. You may need full planning permission, and the council may ask for matching brick, timber windows or a particular roof form. Those materials cost more, so check your council's conservation map early.
Get drawings and structural calculations done before you ask for prices. A builder can only price accurately from a proper specification. Quotes based on a rough sketch tend to rise once the work starts.
Ask each builder to break the price down, so you can compare like for like. Check what is included for foundations, drainage, steels and making good the existing house. Ask how they deal with things nobody can see until digging starts, like an old drain or poor ground. Be wary of any quote much lower than the rest. The gap usually shows up later as extras.
A single storey rear or side return extension usually takes around 10 to 16 weeks on site. Double storey projects often take 4 to 6 months. Allow extra time beforehand for design, approvals and party wall agreements, which can take several months.
Not always. Many single storey rear extensions fall under permitted development, but flats, conservation areas and Article 4 areas have tighter rules. A Lawful Development Certificate from your council confirms that the work is allowed, which also helps when you come to sell.
Once you add stamp duty, estate agent fees, legal costs and removals, moving to a bigger house often costs as much as a good extension, or more. Extending usually makes sense if you like your area and the extra space would add more than it costs, though that varies from street to street.
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