How to Plan a House Extension

Steps to Follow

• Define your goals and budget. Before you speak to anyone, get clear on what you need from the extension and what you can realistically afford. As a rough guide, single-storey extensions in the UK typically cost between £1,500 and £2,500 per square metre, while double-storey builds can be more cost-effective per square metre but require greater upfront investment. Set a contingency fund of at least 10–15% to cover unexpected costs.
• Check permitted development rights. Many single-storey rear extensions in England can be built under Permitted Development (PD) rights, meaning you won’t need full planning permission. However, PD rights don’t apply in conservation areas, Areas of Outstanding Natural Beauty, or for listed buildings. Check with your local planning authority (LPA) or use the Planning Portal (planningportal.co.uk) to verify your rights before proceeding.
• Appoint an architect or designer. A qualified architect or architectural designer will help you maximise your space and produce drawings for planning and building regulations applications. Look for members of the Royal Institute of British Architects (RIBA) or the Chartered Institute of Architectural Technologists (CIAT). Obtain at least three quotes and always check references from previous clients.
• Submit a planning application if required. If your project falls outside PD rights, you’ll need to apply for planning permission through your LPA. The process typically takes eight to thirteen weeks. Your architect will normally manage this on your behalf. Be prepared for neighbour consultation and possible conditions attached to any approval.
• Obtain Building Regulations approval. Separate from planning permission, Building Regulations ensure your extension is structurally sound, energy efficient, and safe. You can use either a local authority building control officer or an approved inspector. Your builder should not start work until you have the correct notices in place.
• Get a Party Wall Agreement if needed. If your extension involves work on or near a shared boundary or party wall, you are legally required to notify your neighbours under the Party Wall etc. Act 1996. Depending on their response, a formal Party Wall Agreement may be needed. A party wall surveyor can guide you through this process.
• Select your builder carefully. Obtain at least three detailed quotes from reputable builders. Look for members of the Federation of Master Builders (FMB) or similar trade bodies. Ask to visit recent projects and speak directly with past clients. Agree a detailed contract covering the scope of work, payment schedule, and completion timeline before any work begins.
• Notify your insurer and mortgage lender. Tell your home insurer about the planned works, as standard policies may not cover building sites. If you have a mortgage, your lender may also need to be informed, particularly for larger structural changes. Failing to do so could invalidate your cover.