Extension Planning in London: A Step-by-Step Guide for Homeowners

Before engaging any professional or builder, be clear about what you are trying to achieve. Common extension goals in London include: creating an open-plan kitchen-diner by extending the rear; adding a utility room or WC at ground floor level; extending laterally into the side return to increase kitchen width; creating a home office or playroom; adding a bedroom at first floor level (double-storey extension); or combining an extension with a reconfiguration of the existing ground floor.

Questions to ask yourself at this stage: what activities will take place in the new space? How many square metres do you need (and is that realistic given your garden and plot)? Do you want the extension to connect seamlessly to the existing interior or be a clearly separate volume? What is your budget — and is that budget realistic for what you want? Do you want natural light from above (roof lantern or skylights) or from the garden-facing wall?

The clearer you are at this stage, the better your architect or designer can translate your goals into a scheme. Vague briefs produce schemes that do not quite hit the target and require expensive revisions.

Permitted Development (PD) rights allow certain types of extension work without needing full planning permission, provided the work stays within defined limits. For single-storey rear extensions in England:

Standard PD limits: an extension can extend up to 4m beyond the original rear wall of a detached house, or 3m for any other house type, without planning permission, subject to conditions (maximum height 4m, no higher than the eaves, no veranda or balcony, materials to match existing).

Larger Home Extensions scheme: a notification process (not full planning permission) allows rear extensions up to 8m for detached houses or 6m for other house types, provided neighbours are notified and raise no objections during the 42-day consultation period.

London-specific restrictions: many London boroughs have Article 4 Directions that remove PD rights in whole or part. Conservation areas in London often have additional restrictions. Some London properties have had PD rights removed by condition when they were originally built or extended. Always check with your Local Planning Authority before assuming PD applies to your property.

How to confirm your PD position: apply for a Certificate of Lawful Development (CLD) from the LPA. This is a formal confirmation that the proposed extension is lawful without planning permission and is recommended before starting any PD extension — particularly in London where PD restrictions are common.

For most London extensions, an architect or architectural technologist is needed to produce the design drawings. These drawings are used for:

- Planning application or Certificate of Lawful Development - Building Control application - Structural engineer coordination - Contractor tendering and pricing - Building Control inspection

Architect vs architectural technologist: architects (ARB-registered) are trained in building design and can provide a broad design service including initial concept, planning, and technical design. Architectural technologists (CIAT-chartered) focus on the technical delivery of buildings and are often more cost-effective for straightforward residential extensions where detailed technical drawings are the primary requirement. For most London rear extensions, an architectural technologist provides excellent value.

What the drawings should include: existing floor plans and elevations (existing state); proposed floor plans, elevations, and sections (showing the new extension in relation to the existing); roof plan; window and door positions and sizes; materials specified; site location plan; block plan showing the extension footprint relative to plot boundaries.

Fee range: architectural drawings for a simple single-storey rear extension typically cost £1,500–£4,000 depending on the designer's experience and the complexity of the project.

Once the architectural drawings are complete, the appropriate planning route is followed:

Certificate of Lawful Development (PD route): where the extension falls within Permitted Development, submit a CLD application to the LPA with the architectural drawings. The LPA has 8 weeks to determine the application. The CLD is not planning permission — it is confirmation that the works are lawful. Cost: typically £206 application fee plus architect's fee to prepare the application.

Full planning application: where the extension requires planning permission (double-storey extensions, extensions in conservation areas, non-PD proposals), a full planning application is submitted. The LPA has 8 weeks (sometimes extended to 13 weeks for complex applications) to determine it. Planning applications are public and neighbours can comment. Cost: £258 application fee (for a single house extension) plus architect's planning submission fee.

Pre-application advice: for complex or sensitive applications, pre-application advice from the LPA's planning officer can save time and reduce the risk of refusal. A fee is charged (varies by LPA, typically £150–£500 for residential proposals).

If planning is refused: the applicant can appeal the decision to the Planning Inspectorate. Appeals take approximately 6 months and have a material cost. It is usually more cost-effective to design to address the planning officer's objections and resubmit than to appeal.

Once planning is in hand (or confirmed as PD), the structural engineering and Building Control application run in parallel.

Structural engineer: appoint a Chartered Structural Engineer to design the steel beam over the rear wall opening, the extension foundation, and any retaining elements. The engineer produces calculations and drawings for Building Control submission. See our structural engineer guide for details.

Building Control application: Building Control approval is required before construction starts on any notifiable works (which includes almost all extensions). Options:

Full Plans application: submit the architectural drawings and structural drawings to Building Control before works start. Building Control reviews the plans and issues an approval. This provides certainty that the design is compliant before you start building. The inspector then carries out stage inspections during construction.

Building Notice: a lighter-touch option where you notify Building Control that works are about to start, without submitting full plans. The inspector visits during construction. This approach relies on the contractor building correctly without pre-approved plans — it offers less certainty and is not recommended for complex structural works.

Building Control fees: typically £300–£600 for a single-storey extension, payable to the LPA or to an approved inspector (private Building Control).

Stage inspections: Building Control will inspect at several stages: commencement, foundation excavation and concrete, damp proof course level, floor slab, structural steelwork (before covering), roof structure, and completion. The inspector must be notified before each stage.

Party wall notices must be served before works affecting the party wall or excavation near neighbouring foundations. For a rear extension in a terraced house:

Section 2 (party wall work): if the extension structure connects to, builds off, or cuts into the party wall, a Section 2 notice must be served on the adjoining owner(s) at least two months before work starts.

Section 6 (excavation): if the extension foundation is within 3 metres of the neighbour's foundation and at a depth lower than their foundation, a Section 6 notice must be served at least one month before work starts.

If the neighbour agrees in writing: work can proceed. If they do not respond or disagree: a surveyor is appointed for each side (sometimes a single Agreed Surveyor serves both parties) and a Party Wall Award is prepared. This takes 4–8 weeks and delays the construction start.

Practical advice: serve party wall notices early — as soon as the structural design is far enough advanced to confirm the foundation and wall details. Do not wait until you have a contractor and a start date, only to discover a party wall dispute delays the project by two months.

With planning, structural engineering, Building Control, and party wall in hand, the contractor selection and build phase can begin.

Getting quotes: provide each tendering contractor with the same set of information — architectural drawings, structural drawings, any specification of finishes, and a clear description of what is included and excluded. Without a consistent tender pack, quotes are not comparable.

What to check when selecting a contractor: evidence of relevant experience (residential extensions in London); references from recent clients (call them); current public liability and employers liability insurance; a clear pricing document that matches the project scope; a programme showing start and completion dates; a clear payment schedule (avoid large upfront payments); and a contract (JCT Minor Works or similar).

Design and build route: RCB Design & Build offers a design-and-build approach where one team coordinates the architectural design, structural engineering, planning, and construction under one management structure. This eliminates the coordination gap between the design team and the building team, reduces the risk of design errors discovered on site, and provides a single point of accountability throughout the project.

Build duration: a single-storey rear extension typically takes 8–14 weeks on site from strip foundations to handover, depending on size, specification, and weather.