A rear or side extension is one of the most effective ways to add space and value to a London home. Here is what you need to know about planning, cost, and choosing the right contractor.
A well-designed house extension can transform how a London property works — creating open-plan kitchen-dining spaces, additional bedrooms, home offices, or ground-floor living areas that meet the demands of modern family life. It is one of the most popular and cost-effective improvements available to London homeowners.
This guide covers the main types of extension, planning rules, cost ranges, and what to look for in a contractor.
Types of house extension
Rear extension — the most common type. Extends the ground floor (or sometimes first floor) at the back of the property. In many cases falls under permitted development.
Side return extension — infills the narrow alley beside a Victorian or Edwardian terrace to create a wider kitchen or living space. A particularly effective transformation for period properties.
Double-storey extension — adds two floors rather than one, maximising the gain from a single planning application and foundation construction. Often more cost-efficient per square metre than a single-storey.
Wrap-around extension — combines a rear and side return extension in an L-shape. Creates a large open-plan space and is increasingly popular in prime London terraces.
Over-garage extension — adds a room above an integral or adjacent garage. Usually permitted development if the garage is at the side of the house.
Planning rules in London
Much extension work in London falls under permitted development rights, meaning no formal planning application is required. The key rules for a single-storey rear extension under permitted development are:
- —Detached houses: extends no more than 8 metres beyond the original rear wall
- —Semi-detached and terraced houses: extends no more than 6 metres
- —Maximum height 4 metres (3 metres if within 2 metres of a boundary)
- —Materials must be similar in appearance to the existing house
Important exceptions and complications:
- —Conservation areas — most prime London postcodes are conservation areas. Extensions visible from the street almost always require planning permission
- —Listed buildings — any extension to a listed building requires listed building consent
- —Flats and maisonettes — have no permitted development rights; all extensions require full planning permission
- —Article 4 directions — some councils have removed permitted development rights in specific areas
For the prior approval route (larger single-storey extensions up to 8m for detached or 6m for others), you must notify the council before starting and obtain their confirmation.
Cost ranges for London extensions, 2026
These figures are all-in costs including design, structural engineering, planning (where needed), and build:
| Type | Typical budget range |
|---|---|
| Single-storey rear extension (mid spec) | £80,000 – £130,000 |
| Single-storey rear extension (high spec) | £130,000 – £200,000 |
| Side return extension | £70,000 – £120,000 |
| Double-storey rear extension | £150,000 – £280,000 |
| Wrap-around extension | £160,000 – £300,000+ |
Specification is the biggest variable after size. A kitchen-dining extension with premium stone floors, bi-fold doors, underfloor heating, and a bespoke kitchen will cost materially more than the same footprint with standard finishes.
What drives the cost
Substructure — foundations are a significant cost, particularly in London's clay soil where movement is a consideration. Strip foundations are standard; raft or piled foundations may be needed on poor ground.
Structural steel — opening up the back of a property almost always requires steel beams. The complexity of the steelwork affects both cost and programme.
Roof design — flat roofs are cheaper to build than pitched roofs. Glazed roofs and lanterns add cost but have a significant impact on light quality.
Glazed elements — bi-fold or sliding doors, roof lanterns, and structural glazing are among the most cost-sensitive items. Specify carefully and get lead times early.
Party wall — any extension close to or on a shared boundary triggers the Party Wall Act. ASAAN manages the surveying and agreement process, but allow 2–3 months before construction can start.
How long does an extension take?
A single-storey rear extension typically takes 4–6 months from planning (where required) to handover. Double-storey extensions take 6–9 months. Add 2–3 months at the front for design, structural engineering, and party wall agreements.
The full process from initial brief to moving in typically takes 9–12 months for a standard extension.
Choosing a contractor
The most important decision in an extension project is who you hire to build it. At this level of investment, the cheapest quote is rarely the right one.
Look for:
- —Experience with London period properties — Victorian and Edwardian terraces have specific structural characteristics that require experience
- —Planning track record — particularly for conservation area applications
- —Party wall experience — a contractor who manages the process rather than outsourcing it
- —Transparent programme and reporting — you should know where your project is at all times
ASAAN works across all of the above. We are happy to visit your property, understand your brief, and give you an honest view of what is achievable and what it will cost. Contact us to arrange a site visit.
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