The London Home Extension Design Process: From Brief to Planning

The brief is the document or conversation that captures what the London homeowner wants to achieve from the extension — not in terms of dimensions or floor plans, but in terms of how the family wants to live and what problems the extension is solving. A strong brief for a London extension covers: lifestyle goals (what activities will happen in the new space? How will the family actually use it day to day?); space needs (how many people, how often, and what circulation patterns?); connection with outdoor space (is connection to the garden or rear yard important? How often will it be used as a connected indoor-outdoor space?); light preferences (how important is natural light? In which direction does the garden face?); character and aesthetic preferences (period-sensitive, contemporary, or a mix?); budget (a realistic construction budget, including contingency — see extension cost guide); and timescale preferences (when does the family want to be in the finished space?). The brief directly influences the design — the more clearly the homeowner can articulate what they want to achieve and how they live, the better the architect can develop a design that genuinely meets those needs rather than producing a generic rear extension. Not all briefs need to be formal documents — some architects work through the brief in initial consultations through structured questions — but ensuring that all family members who will be affected by the extension have agreed on the goals before design work begins avoids expensive design changes later.

Selecting the right architect for a London extension is one of the most important decisions in the project. The architect establishes the design concept, manages the planning application, and (if retained for detailed design) produces the construction drawings that the builder prices and builds from. How to find an architect for a London extension: RIBA Find an Architect (architecture.com): the Royal Institute of British Architects's search tool allows searching for architects by location and project type. Recommended by a friend or neighbour: the most reliable route — seeing an extension that a neighbour has built and being happy with it before approaching their architect. Portfolio review: look for architects with experience specifically in London residential extension projects and in the planning context of your London Borough. Conservation area and listed building experience is important if the property is in a conservation area. Types of practice to consider: small practice or sole practitioner (typically more hands-on principal involvement in a domestic project — the person you meet is often the person who designs the project); medium practice with a domestic residential team (more resource and technical support, but may have less senior principal time on a householder project); design-and-build contractor (some London design-and-build contractors — including RCB Design & Build — offer design coordination as part of the overall project package, integrating design, planning, and construction in one relationship). Architecture fee structures for London extensions: a percentage of construction cost (typically 5%-15% for full service from concept to contract administration); a fixed fee (agreed upfront for specific stages — concept, planning, building regulations — and re-agreed for subsequent stages); an hourly rate (for smaller or more exploratory commissions). RIBA stages for a London extension: a standard RIBA-stage appointment covers: Stage 1 (Preparation and Brief); Stage 2 (Concept Design); Stage 3 (Developed Design); Stage 4 (Technical Design/Building Regulations); Stage 5 (Construction); Stage 6 (Handover). Not all London extension clients appoint to full service — some appoint for Stages 1-4 only (design and planning) and then manage the builder relationship themselves.

The concept design stage develops the initial design ideas for the London extension — testing different layout options, exploring the massing and form of the proposed structure, and establishing the broad design intent. What happens at concept stage: the architect studies the existing property (site visit, measured survey, orientation, existing plans if available); develops 2-3 sketch layout options for the client to review and discuss; tests massing, form, and approximate dimensions against Permitted Development limits and local planning policy; prepares a pre-application advice request where the planning implications are complex. The measured survey: before detailed design begins, the architect commissions (or carries out) a measured survey of the existing property — producing accurate drawings of the existing floor plans, sections, and elevations that form the base drawings for the proposed extension design. In London, where many Victorian terraces were not built to precise dimensions (and where extensions have been added over decades), the measured survey is essential for accurate design. A laser-measured survey by a specialist surveying practice: £500-£1,500 for a typical London terrace. Pre-application advice: for a London extension in a conservation area, or where the proposed design is close to or exceeds Permitted Development limits, a pre-application advice submission to the London Borough before formal planning application is extremely valuable. Pre-application advice allows the homeowner and architect to test the proposed design with the planning department before spending money on a full planning application — understanding the likely officer view and what design adjustments might be required. Cost of pre-application advice: typically £75-£300 for a householder pre-application in London (charged by the Borough). Timeline: most London Boroughs respond to pre-application submissions within 4-8 weeks.

Where the proposed London extension requires planning permission (because it exceeds Permitted Development limits, or because the property is in a conservation area with Article 4 Directions), the architect prepares a planning application. What a planning application for a London extension includes: application form (submitted through the Planning Portal); location plan (a 1:1250 scale map showing the property in context); block plan (a 1:500 scale site plan showing the footprint of the proposed extension in relation to the site boundary and neighbouring properties); existing and proposed floor plans (1:50 or 1:100 scale drawings showing the existing ground floor plan and the proposed ground floor plan with extension); existing and proposed elevations (drawings of the external walls, showing the extension in relation to the existing building and neighbouring properties); design and access statement (a brief document explaining the design principles, the rationale for the extension, and its relationship to the surrounding area — particularly important in conservation areas); photos of the existing property and the surrounding street. Application fees: the planning application fee for a householder application in England (single house, extension) is currently £528 (as of April 2025). The householder application process: London Boroughs are required to determine householder planning applications within 8 weeks of validation. In practice, many London Boroughs take 8-12 weeks, and some complex applications (conservation area, objections received) take longer. During the 8-week period, the Borough consults neighbours (typically a 21-day consultation period) and any statutory consultees (conservation area officer, highways, drainage). Planning approval with conditions: most London extension planning approvals come with conditions that must be discharged before or during construction — for example, material samples to be approved (conditions 3 and 4 in most conservation area approvals); construction management plan; archaeological investigation; hours of construction.

Once planning permission is granted (or confirmed as not required for a Permitted Development scheme), the technical design stage develops the drawings to a standard sufficient for Building Regulations approval and for the contractor to price and construct from. Building Regulations for London extensions: all London extensions require Building Regulations approval — either through a Full Plans application (drawings submitted to the Building Control body for approval before work starts) or a Building Notice (notification of intention to start, with inspection-based compliance rather than pre-approval of drawings). Full Plans route is strongly recommended for any extension of significant scale — it provides certainty on compliance before the build starts and reduces the risk of disputes during construction. Key Building Regulations topics for a London extension: Structure: all structural elements (foundations, walls, floors, beams, roof structure) must meet Part A of Building Regulations. A structural engineer's calculations are required. Thermal performance: extensions must meet minimum U-value requirements under Part L of Building Regulations (walls, roof, floor, glazing). Fire safety: fire doors, escape routes, and fire separation may be required depending on the extent of work (Part B). Drainage: any drainage alterations or additions must comply with Part H. Ventilation: Part F requires adequate background and extract ventilation. Electrical: any new electrical work must comply with Part P (typically certified by a qualified electrician). Building Control fees for a London extension: approximately £600-£1,500 for a standard single-storey rear extension, depending on the London Borough's fees and the value of work. Technical design drawings for a London extension: the technical design package (produced by the architect, structural engineer, and specialist consultants) typically includes: detailed floor plans and sections (1:20 scale); wall sections and construction details (1:5 to 1:10); structural drawings and calculations; window and door schedules; specification document listing materials, products, and standards. This package is submitted to Building Control and issued to the builder for tender and construction.