Rear Extension Guide for London Homeowners

For many eligible houses, a single-storey rear extension may fall within ordinary permitted development limits of 3 metres for an attached house or 4 metres for a detached house, measured from the original rear wall. Height, eaves, materials, garden coverage and all other conditions still apply.

The larger-home-extension route can allow depths up to 6 metres for attached houses and 8 metres for detached houses, subject to the prior approval/neighbour consultation procedure and eligibility restrictions. It is not an automatic extra allowance, does not generally apply on designated land, and is not available to flats. Do not start before the procedure is lawfully completed.

Full planning permission is the route for proposals outside the available rights, or where those rights have been removed. It allows the council to assess the design, neighbours and local policies rather than simply test the permitted development rules.

An extension can leave the former rear room dark. Test rooflights, roof lanterns and the position of solid walls alongside furniture, kitchen storage and circulation. Large glazing areas need an overheating, privacy and energy strategy; more glass is not always better.

Measure levels and locate drains, manholes, trees and access routes before fixing the footprint. Sewer agreements, foundations beside neighbouring structures and awkward builder access can affect feasibility and cost.

For an early feasibility exercise only, a straightforward London extension might be tested at £2,500–£4,000 per square metre excluding VAT, professional fees and major specialist items. This is an illustrative planning allowance, not a published Shorplans project cost or a contractor quotation. A small, high-specification or structurally complex scheme may exceed it.

Allow for survey and design, the appropriate planning or certificate process, structural design, building control, party wall matters and tendering before construction. A non-major planning application commonly has an eight-week determination target after validation, but information requests, extensions of time and local workloads can make the elapsed period longer.

For an initial programme discussion, a relatively straightforward single-storey addition might be tested against a 12–20 week site allowance after design, consents and procurement. That is an illustrative scheduling assumption, not a Shorplans commitment or the documented duration of a portfolio project.

Agree the actual sequence with the chosen contractor, including long-lead glazing, weather protection, structural alterations and kitchen delivery. A larger scheme, difficult access or unexpected ground conditions may need materially longer.

Get a coordinated drawing and specification package, not just planning elevations. Agree drainage, structure, insulation, ventilation, finishes and inspection responsibilities. Protect the existing house from weather and plan how you will live around the work.

Budget separately for VAT where applicable, contingency, temporary accommodation and kitchen or bathroom fit-out. Compare tenders against the same inclusions and exclusions.