The Complete Guide to Building an Extension
Building an extension is one of the most popular ways homeowners add space and value to their properties. Whether you’re creating a larger kitchen, adding a home office, or constructing an extra bedroom, a well-designed extension can transform your living space and enhance your home’s market appeal. However, extending a property involves navigating a complex landscape of planning rules, building regulations, legal responsibilities, and construction logistics.
1. Types of Extensions
Before starting, it’s important to consider the different types of extensions:
Single-Storey Rear Extension
• Built to the rear of the property.
• Often used to expand kitchens, dining areas, or add a utility room.
Side Return Extension
• Makes use of unused space along the side of a property, common in Victorian terraces.
• Ideal for extending narrow kitchens.
Wrap-Around Extension
• Combines rear and side extensions into an L-shaped layout.
• Provides significant additional space and layout flexibility.
Double-Storey Extension
• Adds space on two floors.
• Often includes an extra bedroom and bathroom upstairs and a living area downstairs.
Loft Conversion or Dormer Extension
• Adds living space in the attic area.
• Often used for an additional bedroom or office.
Porch or Front Extension
• Small additions at the entrance.
• Can be for aesthetics, storage, or weatherproofing.
2. Do You Need Planning Permission?
Whether you need planning permission depends on the scale, location, and design of your proposed extension.
Permitted Development (PD) Rights
Under PD, you may not need full planning permission for certain types of extensions, provided you meet specific criteria:
Single-Storey Extensions (England)
• Must not extend beyond the rear wall of the original house by:
• 4 metres for detached houses.
• 3 metres for other houses.
• Must not exceed 4 metres in height.
• Maximum eaves height of 3 metres if within 2 metres of a boundary.
• Must not cover more than 50% of the total land around the original house.
• Must use similar materials to the existing house.
Larger Home Extensions Scheme
• Allows extensions of up to 6 metres (semi-detached/terraced) and 8 metres (detached).
• Requires prior approval from the local authority with neighbour consultation.
Double-Storey Extensions
Usually require full planning permission unless highly restricted.
Other Restrictions
• Listed buildings always require consent.
• Conservation areas, National Parks, and Areas of Outstanding Natural Beauty (AONBs) have tighter controls.
• Article 4 Directions can remove permitted development rights.
Planning Application Process
• Submit to your Local Planning Authority (LPA).
• Include drawings, site plan, and a Design & Access Statement.
• A decision is typically made within 8 weeks.
Tip: Even if you think your project is within PD, it’s a good idea to apply for a Lawful Development Certificate (LDC) to prove compliance.
3. Building Regulations Approval
All extensions—whether permitted development or not—must comply with Building Regulations. These are legal standards covering structural integrity, fire safety, energy efficiency, ventilation, and more.
Key Areas of Building Regulations
• Part A – Structure (e.g., foundations, beams, roof structure)
• Part B – Fire safety (e.g., escape routes, smoke alarms)
• Part C – Site preparation and resistance to contaminants
• Part E – Sound insulation (especially for semi-detached or terraced homes)
• Part F – Ventilation
• Part L – Energy conservation
• Part M – Accessibility (especially relevant for ground floor extensions)
Routes to Approval
1. Full Plans Application – Submit detailed drawings for approval before starting.
2. Building Notice – Quicker, for straightforward builds, but with more risk.
3. Approved Inspector – A private sector alternative to local council building control.
Important: Building work is subject to inspection at key stages, such as foundation pouring, damp proofing, insulation, and final sign-off.
4. Design and Architectural Planning
Hiring an Architect or Designer
A professional designer or architect can:
• Help develop design ideas.
• Create planning and building regulation drawings.
• Maximise light, layout, and flow.
• Liaise with planners and engineers.
Structural Engineers
You’ll likely need one if:
• You’re removing load-bearing walls.
• Building a two-storey extension.
• Adding large spans like bifold doors.
They’ll provide structural calculations required for Building Regulations.
Party Wall Surveyor
If your work affects a shared wall (e.g. terraced or semi-detached house), the Party Wall etc. Act 1996 may apply. You must:
• Serve notice to neighbours.
• Possibly appoint surveyors to agree on a Party Wall Award.
5. Cost Considerations
Extension costs vary depending on location, design, materials, and specification.
Typical Cost Estimates (2025, UK-wide average):
• Basic single-storey extension: £1,800 – £2,500/m²
• High-spec single-storey: £2,500 – £3,500/m²
• Double-storey extension: £2,200 – £3,000/m²
• Loft conversion: £25,000 – £70,000+
Additional Costs
• Architect designer & structural engineer fees (5–15% of construction cost)
• Planning application fees (~£250)
• Building control fees (£500–£1,000)
• Party Wall surveyor (if required)
• Utility relocation or upgrades
• VAT (most work subject to 20% VAT, some may qualify for reduced or zero-rated VAT)
Pro tip: Always budget a 10–15% contingency for unforeseen costs.
6. Legal and Neighbour Considerations
Party Wall Act
You must give at least 2 months’ notice before work begins on a party wall or structure near a boundary. Your neighbour can:
• Consent
• Request changes
• Appoint a surveyor (at your cost)
Right to Light
Even if planning permission is granted, neighbours can seek legal action if your extension significantly blocks natural light to their windows.
Boundary and Access
Confirm boundary lines with the title deed or land registry. Be cautious of:
• Overhanging eaves or gutters.
• Scaffolding on neighbour’s land (requires consent).
7. Managing the Build
Choosing a Builder
• Get at least three quotes.
• Ask for references and check previous work.
• Ensure they have public liability insurance.
Contracts
Use a written contract, such as the JCT Homeowner Contract, to define:
• Timelines
• Payment schedules
• Responsibilities
• Dispute resolution
Project Management Options
• DIY Management: Cheaper but time-consuming and riskier.
• Main Contractor: Takes on full responsibility for the build.
• Design and Build Company: One-stop-shop offering design, planning, and construction.
8. Completion and Certification
Once your extension is finished, you must:
• Obtain a completion certificate from Building Control.
• Update your home insurance.
• Notify your mortgage provider, if relevant.
• Update the Energy Performance Certificate (EPC) if significant changes have been made.
If you plan to sell your home in the future, having the right paperwork (planning consent, building regulation certificates, party wall agreements) is essential to avoid delays or lost sales.
Conclusion
Building an extension in the UK is an exciting but complex undertaking that involves navigating planning policies, design constraints, legal responsibilities, and regulatory compliance. With the right preparation, professional advice, and careful budgeting, an extension can greatly enhance both the usability and value of your home.
Start with a clear plan, engage the right experts, and maintain good communication with neighbours and contractors throughout the process. Whether it’s a simple rear extension or a substantial two-storey addition, the key to success is informed decision-making at every stage.
Disclaimer: This article is for general guidance only and does not constitute legal, financial, or professional advice. Always consult qualified professionals and local authorities before starting any building project.