House Extensions

Each type has its own structural, planning and cost logic. The dedicated pages go into detail; in brief:

• Rear extensions — the most common. A single-storey addition across the back of the house, usually for a kitchen-dining space, with large glazed doors to the garden.
• Side return extensions — infilling the narrow passage beside a terraced house to widen the kitchen. Small in area, large in effect.
• Wraparound extensions — a rear and side extension combined into one L-shaped space; the biggest ground-floor transformation.
• Double storey extensions — adding living space below and a bedroom or bathroom above; the most cost-efficient way to add area per pound.
• Kitchen extensions — any of the above, designed around the kitchen as the centre of the house.

Many single-storey extensions in England can be built under permitted development rights without a planning application, provided the house is not a flat, is not in a conservation area with an Article 4 direction, and has not already used its allowance. The key limits, taken from the Planning Portal (Town and Country Planning (General Permitted Development) (England) Order 2015, Class A):

"Original house" means the house as first built or as it stood on 1 July 1948. Previous extensions count against the allowance. On designated land — conservation areas such as Hampstead Garden Suburb, Totteridge, Mill Hill and Monken Hadley in Barnet, or Harrow on the Hill — side extensions and two-storey rear extensions need planning permission, and Article 4 directions can remove permitted development altogether. Flats and maisonettes have no permitted development rights for extensions.

Wraparounds, double-storey extensions on most houses, anything on a flat, and anything in an Article 4 area will need a householder planning application (fee £258 in England; the council has eight weeks from validation to decide). Even where a scheme is permitted development we recommend applying for a Lawful Development Certificate (£129) so there is formal proof when you sell or remortgage. We handle all of this as part of the design stage — see architectural design and planning.

Sources: Planning Portal, "Extensions — planning permission"; Planning Portal fee schedule (England, April 2025). Rules change; we confirm the position for your property at the site visit.

Every extension needs Building Regulations approval regardless of planning. This covers foundations (in North London's clay, often 1 m or deeper, and deeper still near trees), structural steel where the rear wall is opened up, thermal performance of walls, roof and glazing (Approved Document L), drainage, ventilation, electrics and fire safety. We submit a full plans application with structural calculations so the design is approved before work starts, and building control inspects at foundation, drainage, structure, insulation and completion stages.

Because most North London extensions sit on or near a boundary, the Party Wall etc. Act 1996 usually applies: building on the line of junction, cutting into a party wall to bear a steel, or excavating within 3 m of a neighbour's foundations (6 m in some cases) all require notice — two months' notice for work to a party wall, one month for new walls and excavation. Neighbours who dissent, or who do not reply within 14 days, trigger the appointment of surveyors and a party wall award. We advise on notices early so it never holds up the programme.

• Light in the middle of the plan. A deep rear extension pushes the original rooms further from the garden. Rooflights over the old rear wall line, a side window, or a glazed roof strip keep the centre of the house bright.
• Ceiling height and floor levels. Aim for 2.5–2.7 m clear; drop the floor if the garden is lower rather than stepping down inside.
• Junction with the old house. Where the new steel meets the old wall is where extensions look either seamless or bolted on. Decide early whether the extension should match the house or be deliberately different.
• Overheating. Large south- or west-facing glazing needs shading, opening rooflights or solar-control glass; Building Regulations now take summer comfort seriously.
• Services. Moving the kitchen means moving drainage, gas and often the boiler. Plan the utility room and the plant at design stage, not on site.
• The garden that is left. A 6 m extension on a 12 m garden leaves very little. Sometimes a smaller extension plus a loft conversion is the better answer.

Site visit and feasibility, then design and the planning route, then technical drawings and a fixed price, then construction — typically 10 to 16 weeks on site for a single-storey extension and 14 to 24 weeks for a double storey. The process page sets out all ten stages and what you receive at each. Most extensions can be built with the family living in the house; we seal the site from the rest of the home and keep a temporary kitchen running where we can.

• Commissioning planning drawings before anyone has priced the idea.
• Ignoring the 50% curtilage rule, previous extensions or an Article 4 direction, then being refused.
• Under-specifying foundations near trees, which building control will not accept and which costs more to fix mid-dig.
• Choosing the kitchen after the drainage has been laid.
• Leaving neighbours until the week before work starts — party wall disputes are the most common cause of delay we see.
• Allowing no contingency. We recommend 10% on a straightforward extension and 15% on an older house.