For a typical house extension in the UK, architects usually charge either a percentage of the construction cost — commonly 7–15% for a full service — or a fixed fee for individual stages. A concept-and-planning drawings package for a straightforward extension often falls somewhere in the low thousands; a full service through to construction costs more.
What you pay for at each stage
- Concept design — initial sketches and options for your space.
- Planning drawings — the drawings submitted with a planning or lawful-development application.
- Building regulations / technical drawings — the detailed package a builder works from.
- Tender and construction support — pricing the job and overseeing the build (optional).
You don’t have to buy every stage from one architect. Many homeowners pay for concept and planning drawings, then use a separate technical designer or a design-and-build contractor for the rest.
How to keep architect costs down
- Turn up with a clear brief and a budget — indecision is expensive.
- Check whether your project is permitted development first, which can simplify the drawings needed.
- Visualise the idea before you commission a full package, so you’re briefing from something concrete.
Before you commit to fees, Unformed’s Building Works render turns a photo of your house into a photoreal view of the finished extension — a fast, low-cost way to test the idea and brief your architect well. It complements technical drawings, it doesn’t replace them. See also how to visualise your extension from a photo and whether you need planning permission.
This guide is general information, not legal or planning advice. Planning rules change over time and are applied differently by each local planning authority (LPA). Always confirm the position with your council before starting work. Unformed’s Planning Review gives an AI-assisted risk assessment of a scheme but does not guarantee any planning outcome.