How to Choose Between Different Extension Types for Your London Home
When London homeowners talk about "adding an extension," they're often thinking of a single image — a glass rear extension opening onto the garden. But there are multiple extension types, each with distinct implications for planning, cost, structural complexity, and what you actually gain. Here's a guide to the main options.
Single-Storey Rear Extension
The most common extension type in London. A single-storey addition to the rear of the house, typically extending the kitchen, dining, or living area and opening to the garden through glazing.
Best for: improving ground-floor living space; kitchen-dining reconfiguration; creating a connection between house and garden.
Planning: often within Permitted Development (up to 3m for semis and terraces, 4m for detached under standard PD; up to 6m/8m under the larger home extension scheme). Conservation areas and Article 4 directions reduce or remove PD rights.
Cost range: £50,000–£120,000 depending on size, specification, and structural complexity.
Double-Storey Rear Extension
An extension on two floors at the rear — adding space to the ground floor and creating an additional room or extending an existing room on the first floor. More structurally complex and more likely to need full planning permission.
Best for: adding both a ground-floor kitchen-dining area and an additional bedroom or bathroom above; maximising value per pound of construction cost compared to two separate single-storey projects.
Planning: almost always requires full planning permission. Impact on neighbouring properties (overlooking and overshadowing) is assessed carefully.
Cost range: £80,000–£180,000+ depending on size and specification. Generally represents better value per square metre than two separate single-storey extensions.
Side Return Extension
Common in Victorian and Edwardian terraced and semi-detached houses, where the original footprint included a narrow side passage between the house and the boundary. Converting this space into the main building adds width to the ground floor — often creating the space needed for a properly sized kitchen.
Best for: Victorian and Edwardian terraces with a side return; adding width to the kitchen; creating a more open ground-floor plan without extending deep into the garden.
Planning: often within PD if within the specified height and width limits, though wall-to-boundary position may require a party wall agreement with the neighbouring property.
Cost range: £35,000–£80,000 depending on size and specification. Often combined with a rear extension to form an L-shaped ground-floor extension.
Wrap-Around Extension
A combination of a side return extension and a rear extension creating an L-shaped or full wrap-around addition. This typically produces the most significant transformation of a ground floor, providing a large open-plan kitchen-dining-living space with garden connection.
Best for: maximum ground-floor transformation; creating a full open-plan rear of the house; properties with both a side return and garden depth to use.
Planning: almost always requires full planning permission due to combined scale.
Cost range: £90,000–£200,000+ depending on size, specification, and structural complexity.
How to Choose
Start with what you need — what problem are you trying to solve? Then overlay the planning constraints for your specific property and area, and then the budget. The answer is usually whichever option best solves your brief within your planning constraints at a cost you can finance.
RCB helps clients through this analysis as part of our pre-construction process — before any architect fees are committed.
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