An extension can make a home work better, but the build is only one part of the project. Before you ask for prices, define what the extra space should achieve, how it connects to the existing rooms, and which permissions or technical checks may affect the design. This checklist is a starting point for UK homeowners; rules differ by nation and property.
1. Write a brief around daily use
Start with the activity the extension must support: cooking and eating, an accessible bedroom, a quiet workspace, or a better route to the garden. Record what feels awkward now and what a successful layout would change. A larger room is not automatically a better one if circulation, daylight or storage are poor.
- Sketch where people enter, cross and leave the new space.
- List furniture, appliances and storage with approximate dimensions.
- Think about daylight, privacy, overlooking and views from neighbouring properties.
- Decide which existing rooms must remain usable during construction.
2. Check the property before fixing the shape
Measure the plot and the existing building, then look at levels, drainage runs, trees, access and nearby boundaries. Ask a designer or structural professional to review walls or foundations that may be affected. Hidden conditions are a common reason for later changes; allow time and money to investigate them before the design is finalised.
Consider how materials and machinery will reach the site, where waste will go and whether the builder can work safely without disrupting access to the home. The route for new heating, electrical, water and ventilation services should be part of the design brief, not an afterthought.
3. Check planning and building control separately
In England, some house extensions may fall within permitted development rights, subject to limits and conditions. The Planning Portal extension guide explains the starting rules. Rights may be limited by the type or history of the property, its location and local restrictions. Flats and listed buildings need particular care. Check the position with your local planning authority before relying on a general guide.
Planning permission and building regulations approval answer different questions. The Planning Portal says most extensions need building regulations approval, even where planning permission is not required. Also ask whether party wall procedures, drainage consents or other approvals are relevant. Scotland, Wales and Northern Ireland have their own systems, so use the authority for your property.
4. Design the junction with the existing home
The best extension often improves the rooms behind it. Review how the new roof joins the house, whether an existing room loses daylight, and how the heating and ventilation strategy will work across both spaces. Think through thresholds, floor levels, door openings and the finish where new and old materials meet.
Decide early whether the project includes a new kitchen or bathroom, major landscaping, replacement windows or redecorating connected rooms. Those choices change the true scope and should appear in the budget.
5. Build a specification before seeking quotes
A drawing shows the shape; a specification explains what is to be supplied and finished. Record the intended wall, roof, window, door, insulation and interior finish standards with your designer. Agree who designs and signs off structural details and who submits the building control application. Give every bidder the same pack so you can compare the same work.
- Ask for an itemised price and clear list of exclusions.
- Separate provisional allowances from fixed selections.
- Check whether VAT, waste, making good and final certificates are included.
- Ask how design changes and hidden issues will be priced and approved.
Our guide to comparing builders’ quotes helps you check the detail behind each total.
6. Plan the sequence and the handover
Agree a realistic order for design, approvals, ordering, site setup, structural work, weatherproofing, services, finishes and inspections. Confirm which decisions you must make before each stage. A simple schedule should show when the home may lose access to a room, garden or service and what temporary arrangements are needed.
At handover, collect the documents relevant to the work: completion certificates, product instructions, warranties, test results and a record of agreed changes. Walk through the finished space with the contractor and document any outstanding items and when they will be resolved.
7. Keep the budget flexible
Use a budget with separate lines for design, surveys, approvals, construction, fittings, finishes and a contingency. The free project budget planner can organise those lines, but it cannot predict the condition of a specific property. Revisit the total when the design and specification become firmer.
A well-planned extension begins with a clear outcome and a checked scope. Our wider renovation planning checklist provides a useful structure for the decisions that follow.