The best designer for your extension is someone who can deliver the services your house and budget need. A planning drawing package, an architectural appointment through construction and a design-and-build contract cover different responsibilities. Compare those responsibilities before comparing the price.
This guide is published by Planning Approvals, which also offers planning design services. The services below are examples to investigate, not an independent league table or a claim that one provider is best for everyone. Descriptions reflect the providers' own published information checked on 9 September 2026; obtain a current written proposal for your project.
London extension services to consider
| Provider | What its published service describes | What to establish in your quote |
|---|---|---|
| Resi | Measured surveys, design, planning support, building-regulations packages and builder introductions. | Which package applies to your extension, who visits the site and which specialist services cost extra. |
| Architecture for London | Residential extensions, heritage refurbishment and design and construction services, with an emphasis on low-energy buildings. | The architectural stages, construction responsibilities and energy-performance work included in the appointment. |
| Extension Architecture | Extension design, planning applications, building-regulations drawings and project-management services. | Whether your fee includes technical design, structural engineering, tendering or construction-stage support. |
| DeVis Architecture | Residential architectural services, including home extensions. | The deliverables for your house, the named project lead, fee stages and site involvement. |
The providers' descriptions establish services they advertise, not independently verified outcomes. Ask to see completed work similar to yours and speak to recent clients where possible. A portfolio image alone cannot tell you the final cost, the designer's exact role or the planning difficulties encountered.
Planning Approvals offers an alternative for homeowners seeking an agreed planning-stage service. Our pricing page explains eligible project types, payment on approval and exclusions. Building-regulations work and council and Portal charges are separate. Compare this defined planning scope with the equivalent stage in another provider's proposal, rather than with its price for a whole project.
Decide what you need help with
Start with a short brief: the rooms you need, the budget available for the whole project, your preferred start date and what matters most about the design. Include photographs, any existing plans and the property's planning history.
For a straightforward rear extension you may want help testing layouts, preparing planning drawings and submitting the appropriate application. A whole-house renovation may also need coordinated structural and services design, a detailed specification, tender documents and someone administering the building contract. A complex site can need specialist input even when the proposed extension is small.
Do not assume that knowing the footprint means the design is already resolved. Daylight, circulation, drainage, structure and the connection to the existing house can materially affect the result. Equally, commission extra services because the project needs them, rather than because a package name sounds comprehensive.
Check the person and the appointment
If a provider uses the title architect, check the named individual on the Architects Register. ARB registration and RIBA membership are different things. The Architects Registration Board's hiring guidance explains checks to make before an appointment.
An architectural designer or technologist may also provide suitable services. Assess their qualifications, competence, relevant experience, insurance and proposed responsibilities. Neither a job title nor a low fee proves suitability. Ask who will actually prepare and check your drawings, and who answers questions if the lead designer is unavailable.
Before paying, obtain a written appointment covering the services, charges, payment stages, responsibilities, exclusions and arrangements for changes or ending the appointment. Ask how complaints are handled and what insurance applies to the proposed work.
Compare fees on the same basis
Send the same brief to a small shortlist and ask each provider to complete this scope check:
| Item | Question to ask |
|---|---|
| Survey | Is a measured survey included, and what is its scope? |
| Design | How many options and revision rounds are included? |
| Planning | Who selects the application route, submits it and responds to the council? |
| Supporting evidence | Are heritage, tree, flood or other specialist reports included or separately appointed? |
| Technical information | Are building-regulations drawings, structural calculations and specifications included? |
| Building contract | Does anyone organise tendering, inspect work or administer the contract? |
| Price and changes | What is payable including any VAT, when is it due, and how are additional services charged? |
Percentage fees need a defined cost basis and a clear explanation of what happens when the project budget changes. Fixed fees need an equally clear scope. Neither automatically costs less overall. See our architect fees guide and the separate application costs guide.
Local practice or an online service?
Compare working arrangements rather than assuming that one model is always better. Ask who surveys the house, how design discussions happen, when site visits are available and how construction queries are handled. A remote design workflow can still include site work; a nearby office does not guarantee regular attendance.
Where the house is listed or in a conservation area, ask for evidence of relevant work and an explanation of the likely consent route. Borough experience can be useful, but no provider can promise approval through relationships with officers. Our conservation-area extension guide explains issues to investigate.
Questions to ask before choosing
- Can you show a comparable completed extension and explain your role?
- What is the biggest unresolved risk in this brief?
- What will I receive at each stage, and what will the builder still need?
- What assumptions underpin the fee and programme?
- Who coordinates the other consultants, and who is responsible for construction-stage decisions?
- What happens if permission is refused or I change the design?
Choose the proposal whose scope, communication and relevant experience fit your project. Keep a written record of the agreed brief and budget, then review changes against it. If the planning route is still unclear, start a property check before committing to a detailed design appointment. The check is an initial screening step, not a grant of permission.
Frequently asked questions
Can a new designer take over drawings I already have?
Show the prospective designer the drawings and your existing appointment. Establish what permission you have to use or adapt the documents, what information can be transferred and what the new designer needs to check or redraw. Agree that review work and its fee before assuming an existing drawing package removes the need for a survey or design assessment.
How can I check a portfolio if a practice cannot share client contact details?
Ask for an anonymised explanation of a comparable project: its scope, the practice's role, the design constraints and which stages it completed. Where available, compare that explanation with the public planning documents. Respect former clients' privacy; an approval record can help establish the planning history but does not prove construction quality or the final cost.