How much does a house extension cost in London?
If you're planning an extension in London, cost is almost always the first question. It's also one of the harder ones to answer, because extension prices vary significantly depending on your property type, how much structural work is involved, and what level of finish you want.
This guide gives you realistic 2026 figures to plan with before you start getting quotes. For most London homes, a house extension cost falls somewhere between £30,000 and £180,000 or more. A single-storey rear extension, which is the most common starting point, typically comes in between £30,000 and £80,000.
Extension cost by type in London
The extension type you build has the biggest single impact on cost. Below are guide ranges for the most common options in East and North London. All figures are subject to a site survey and your specific requirements.
Rear extension:
£30,000–£80,000
The most popular extension type in London. A single-storey rear extension adds floor space at the back of the property, typically creating a larger kitchen, an open-plan kitchen-diner, or a better connection to the garden. The range is wide because the cost depends heavily on how much structural work is required, the size of the extension, and your kitchen and glazing specification.
Side return extension: £35,000–£90,000
Common in Victorian and Edwardian terraces, where a narrow alley runs along the side of the property. Infilling this side return widens the ground floor and can transform a cramped kitchen into something properly usable. The structural work involved in bridging the existing side wall and foundations pushes the base cost slightly higher than a straightforward rear extension.
Wraparound extension: £50,000–£120,000
A wraparound combines a rear extension and a side return in one build. It creates the largest ground-floor transformation available to most terraced houses, typically resulting in a big open-plan kitchen-diner with space for a utility room, downstairs WC, and strong garden connection. The higher house extension cost reflects the increased footprint, additional steelwork, and longer build programme.
Double-storey extension: £80,000–£180,000+
Adds space on both the ground and first floors, making it the most cost-effective option per square metre when you need multiple rooms. Typically used to create a larger kitchen-diner downstairs alongside new bedrooms or bathrooms above. The structural complexity and the requirement for planning permission (as opposed to permitted development for single-storey) contributes to the higher cost.
Extension cost per m2 in London
What affects the cost of a house extension in London?
As a rough guide, extension cost per m2 in London sits between £2,000 and £3,500 for a standard build. High-specification finishes, complex structural work, or premium glazing can push that figure higher. Cost per m2 is useful for rough budgeting but can be misleading on smaller extensions, where fixed costs like foundations, steelwork, and connection to the existing house are spread across fewer square metres. A 15m2 rear extension will typically cost more per m2 than a 30m2 one, even with an identical specification.
Several factors move the final figure beyond the basic extension type and size. Structural steelwork is one of the biggest variables: if load-bearing walls need removing to create open-plan layouts, the cost of the steel beam and associated structural work can add thousands. Foundation conditions vary across London's clay soils, and some properties require more extensive groundwork than others. Your choice of glazing has a significant impact, particularly on rear and wraparound extensions where bifold doors, sliding doors, or a full-width glass wall are common. Kitchen and bathroom specification is another major variable, since the extension often triggers a full kitchen refit at the same time. Access and logistics also matter in denser areas of East and North London, where parking and scaffolding can add cost.
What does a house extension quote include?
How long does a house extension take?
A full extension quote from LLAC covers structural work, foundations, roofing, insulation, windows and external doors, first and second fix, plastering, and decoration. What varies between builders is whether the kitchen, bathroom, bespoke joinery, and external landscaping are included or treated as separate allowances. When comparing extension quotes, make sure you are comparing like for like. A lower headline figure can simply mean more items have been excluded. Ask each builder to confirm what is and is not in scope before you compare numbers.
Build time depends on the extension type and scope. A single-storey rear extension typically takes 10 to 16 weeks on site. A wraparound or double-storey extension takes longer, often 16 to 24 weeks, due to the additional structural work and build programme. Your overall timeline should also factor in any planning permission required, which can add several months before work begins.
Is a house extension worth the cost in London?
For most homeowners, yes. A well-specified extension in East or North London tends to add more value to the property than it costs, particularly when it creates a genuinely better ground-floor layout. The strongest returns come from extensions that address a real functional problem, such as a dark and cramped kitchen, and that result in the kind of open-plan space buyers actively look for in this market. Compared to moving to a larger property in London, where stamp duty, legal fees, and the price gap can easily run to several hundred thousand pounds, extending is often the more practical and financially sensible route.
Get a house extension quote in London
LLAC works with homeowners across East and North London, including Hackney, Waltham Forest, Islington, and Haringey. Whether you have a clear brief or are still working out what is possible for your property and budget, the next step is a conversation.