A successful rear extension starts with the way you want to live: a brighter kitchen, space to dine or a better connection to the garden. Establish the design, approvals and budget before booking the build.
Horizon Architecture · Updated 17 October 2026 · England
Typical fee questions: what does the package cover?
Horizon’s combined rear-extension planning and Building Regulations drawing package starts from £1,750, subject to scope. Standalone planning drawings start from £1,400 and measured surveys from £395. These published starting prices are not a London market average or a full-service construction appointment.
Ask whether the survey, revisions, submission, structural coordination, specifications and site-stage services are included. VAT where applicable, council charges, Building Control, structural engineering and other third-party fees are additional. Read the 2026 architect fees guide and published fee table.
The process, from first conversation to site
| Stage | Main work | What allows the next step |
|---|---|---|
| 1. Brief and survey | Agree rooms, budget and priorities; measure the house and review constraints. | Reliable existing information and an agreed brief. |
| 2. Design options | Test layout, daylight, garden connection and neighbour relationships. | Your approval of a developed design. |
| 3. Planning route | Prepare a planning application, lawful development certificate or prior-approval submission as appropriate. | The relevant decision or confirmed lawful route. |
| 4. Technical design | Coordinate structure, drainage, insulation and Building Regulations information. | A suitable coordinated package and the required approvals. |
| 5. Pricing and preparation | Obtain comparable contractor prices and agree scope, responsibilities and programme. | A contract, resolved pre-start matters and an agreed start. |
| 6. Construction | Build with inspections and the agreed level of professional support. | Completion checks and required records. |
How long does a London rear extension take?
Build the programme around decisions and dependencies. Survey availability, your design revisions, council validation, specialist input and contractor lead times all affect the start date. Design preparation takes place before the council’s assessment period.
The normal statutory determination period for a non-major planning application is eight weeks from a valid application, unless a longer period is agreed or another statutory category applies. Validation delays are additional, and a decision within eight weeks is not guaranteed. See government guidance on determination periods.
Technical design, tendering and pre-start arrangements need their own programme. Construction duration depends on the extension’s scale, structural work, access, procurement and whether you remain in the house. Request a project-specific schedule instead of treating a generic number of weeks as a commitment.
Design choices that matter in London
- Daylight: consider the existing middle rooms as well as the new extension.
- Boundaries: test eaves, parapets, glazing and the relationship with neighbours.
- Structure: understand the implications of removing rear walls and creating wide openings.
- Drainage: locate drains and sewers before fixing foundations and layouts.
- Comfort: balance rooflights and glazing with ventilation and overheating control.
- Access: discuss deliveries, storage and working arrangements for a constrained site.
The largest footprint is not always the most useful layout. A well-positioned opening or rooflight may make a smaller extension work better.
Planning permission or permitted development?
Some rear extensions qualify for permitted development; others need prior approval or a full planning application. Flats, conservation constraints, planning conditions and previous additions can change the route. Read our permitted development guide before relying on a maximum depth.
Consent for the design does not replace technical coordination. Our Building Regulations service explains the next drawing stage.
Budget before choosing the final specification
Use the construction cost calculator to explore an initial range. Then check construction, professional fees, VAT, statutory charges, kitchen and finishes, temporary arrangements and contingency against its included allowances. Avoid counting an allowance twice.
For design support, visit our rear-extension architect service. Bring the address, photographs and a realistic overall budget to the first conversation so the brief can be shaped around your priorities.
Frequently asked questions
Do I need an architect for a small rear extension?
The level of professional support should match the risks and scope. Even a small extension can involve structure, drainage, neighbour relationships and technical approvals.
Can a builder price planning drawings?
They may offer an initial estimate, but planning drawings usually do not define every construction detail or specification. Comparable tenders need an agreed, sufficiently detailed package.
Does the drawing package include site visits?
Do not assume so. Ask for site-stage services, inspection arrangements and contract administration to be explicitly included in the appointment if required.
Plan your rear extension with Horizon
Tell us what you want to change and share the property address, photographs and any existing drawings.
Get an extension quoteSources and scope
Guidance checked 17 October 2026. General information for homes in England; confirm the current requirements and agreed fees for your property before proceeding.
