Barnet is one of London's largest residential boroughs, stretching from East Barnet and New Barnet in the north down to Finchley, Hendon, Mill Hill, and Edgware. The housing stock is predominantly interwar suburban — large semi-detached and detached houses with substantial rear gardens — which makes the borough one of the most active for both rear extensions and side-return additions. RCB Design & Build covers EN4, EN5, N2, N3, N12, NW4, NW7, NW9, and surrounding areas, working on extensions from design stage through to completion under a single principal contractor contract.
Extension Types Common in Barnet
The most frequently built extension in Barnet is the rear single-storey kitchen extension, typically on a 1930s or 1950s semi where the original kitchen is in a small back addition. These extensions bring the kitchen forward into a larger footprint, often with bifold doors across the full rear elevation and a flat or lantern-lit roof. A standard rear extension of this type — 4 to 5 metres deep and the full width of the house — typically costs between £55,000 and £75,000 fully fitted and decorated, depending on the kitchen specification and quality of finishes.
Side-return extensions are less common in Barnet than in inner London (the plots tend to be wider, so the side return is not the constraint it is in Islington or Hackney), but they are used on detached properties to create utility rooms, home offices, or wraparound kitchen extensions where the kitchen runs along both the rear and the side of the house.
Double-storey rear extensions are popular in Barnet because many of the larger semis and detached houses have planning history that already established the rear building line. Adding a second storey to an existing ground-floor addition is often structurally straightforward and delivers significant additional bedroom or bathroom space. These projects require full planning permission and take eight to twelve weeks to determine at Barnet's planning department.
Planning in Barnet: What to Know Before You Submit
Barnet operates standard permitted development rules for single-storey rear extensions, with the Prior Approval route allowing up to eight metres on a detached house and six metres on a semi. However, the borough has a number of Article 4 directions in specific streets and conservation areas — Totteridge Village, parts of East Barnet, and areas around Finchley are worth checking before assuming permitted development applies. Properties in or adjacent to the Green Belt (which touches the northern edge of the borough) also carry additional restrictions on the cumulative increase in footprint.
Barnet planning officers generally respond well to applications that are well-drawn, include a clear design rationale, and address neighbouring amenity impacts specifically. Poorly prepared submissions tend to generate requests for further information, adding two to three months to the approval timeline.
Structural Considerations on Barnet Extensions
Many properties in North London were built on London Clay, which means foundations for extensions need to be designed to account for clay shrinkage and heave — typically requiring deeper strip foundations than would be needed on chalk or gravel sub-strata. In streets close to trees, the proximity of root systems to the proposed foundation is a material consideration, and structural engineers will specify accordingly. A contractor with local experience will not be surprised by these requirements and will have priced for them correctly rather than returning with a post-contract variation.
RCB Coverage in Barnet and North London
RCB Design & Build works across Barnet, Finchley, Hendon, Edgware, Mill Hill, East Barnet, and New Barnet, as well as neighbouring Enfield and Haringey where required. The company holds FMB, TrustMark, and Checkatrade accreditations and operates as a full principal contractor — managing design, planning, structural engineering, and construction under one agreement.
Planning an extension in Barnet or North London?
Call +44 7359 872594, email contact@rcbgroup.co.uk, or visit rcbgroup.co.uk to book a site visit and initial consultation.