A single-storey rear extension is one of the most common home improvement projects in the UK — and one of the most transformative. Done well, it can add significant living space and value. Here's how to approach the planning process from the beginning.
Step 1: Understand permitted development
In England, most single-storey rear extensions can be built under Permitted Development (PD) — without requiring a full planning application. The key rules are:
- Maximum depth of 3 metres for terraced or semi-detached houses (or 4 metres under the Larger Home Extension scheme with neighbour notification)
- Maximum depth of 4 metres for detached houses (or 8 metres under the scheme)
- Maximum height of 4 metres (or the height of the existing eaves if lower)
- No more than half the area of the original curtilage covered by extensions or outbuildings
Note that Article 4 directions can remove PD rights in conservation areas and some designated zones. Always check with your local planning authority before assuming PD applies.
Step 2: Sketch your ideas in Free Room Planner
Before approaching an architect, sketch your ideas. What rooms are you creating? How does the extension connect to the existing house? What are the approximate dimensions?
Draw the existing floor plan in Free Room Planner, then extend the walls to show your proposed addition. This gives you a visual reference to discuss with an architect and helps you think through practical questions — where does the new space connect? Is the kitchen being extended, or is a new room being added?
Step 3: Engage an architect or designer
For a single-storey extension, you can work with a full architect, an architectural designer, or a specialist extension company. Get quotes from at least three. Share your Free Room Planner sketch as a starting point — it saves time and communicates your intentions clearly.
Step 4: Structural considerations
Any extension will require a structural engineer to design the foundations and any steelwork for new openings into the existing house. This typically costs £300–800 for straightforward extensions. The architect will coordinate this as part of the design process.
Step 5: Building regulations
Even if your extension doesn't require planning permission, it will need to comply with Building Regulations. You'll need to submit either a Full Plans application (reviewed before work starts) or a Building Notice (inspected during work). Your architect or builder will guide you through this.
Step 6: Get multiple contractor quotes
With architectural drawings in hand, approach at least three contractors for quotes. Share the drawings along with a written scope of works. Prices vary significantly — a like-for-like comparison requires identical scope documents.
Step 7: Use a floor plan to brief everyone
Throughout the process — with the architect, structural engineer, main contractor, kitchen fitter, and electrician — having a clear floor plan prevents miscommunication. Export your Free Room Planner plan at each stage to keep everyone working from the same document.
Typical costs (England, 2025)
- 5m² extension: £25,000–£40,000
- 15m² extension: £50,000–£80,000
- 25m² extension: £80,000–£120,000
These are indicative ranges — costs vary significantly by region, specification, and contractor. Always get multiple quotes.