What You Need To Know Before Starting A Rear Extension

Starting a rear extension project is an exciting step towards transforming your home, giving you much-needed extra space and potentially adding significant value. However, it’s a major undertaking that requires careful planning and research. Before you dive into blueprints and builders, here is everything you need to know, presented in good old British English.

1. 📋 Planning Permission vs. Permitted Development

The very first hurdle you need to clear is determining whether your proposed extension requires a full Planning Permission application or if it falls under Permitted Development (PD) rights.

• Permitted Development: Many smaller, single-storey rear extensions can be built without a formal application, provided they adhere to strict limits on size, height, and proximity to boundaries.

  • Limits: Typically, single-storey rear extensions cannot extend further than 3 metres from the original rear wall for a terraced or semi-detached house, or 4 metres for a detached house.

  • Key Point: Even if your extension is PD, it’s highly recommended to apply for a Lawful Development Certificate from your local council. This proves the work was legal when completed, which is crucial when you eventually come to sell the property.

• Planning Permission: You will definitely need this if your extension exceeds the PD limits, is on the side of the house, is two storeys, or if your property is in a designated area like a Conservation Area, a National Park, or is a Listed Building.

Note: Rules are slightly different in Wales, Scotland, and Northern Ireland, so always check the specific guidance for your region.

2. 🧱 Building Regulations Approval

Crucially, Permitted Development does not exempt you from Building Regulations. This is a separate, non-negotiable legal requirement that ensures your extension is structurally sound, safe, energy efficient, and meets fire safety standards.

• What it Covers: Foundations, damp-proofing, wall structure, roof integrity, ventilation, electrical wiring, and drainage.

• The Process: You can submit a Full Plans Application (recommended) or a Building Notice. A Building Control Officer will inspect the work at various stages (e.g., when the foundations are dug, before plastering, and upon completion).

• Sign-Off: You will receive a Final Certificate upon successful completion, which is another document your solicitor will request when you sell the property.

3. 🏡 The Party Wall Act 1996

If your extension involves building on or near a boundary wall shared with a neighbour (a Party Wall), you must adhere to the Party Wall Act 1996. This is often the trickiest part of the process.

• The Trigger: The Act is usually triggered if you are:

  • Building astride the boundary line.

  • Excavating foundations within 3 metres of your neighbour’s structure and deeper than their foundations.

  • Cutting into a shared wall (e.g., to insert a steel beam).

• The Process: You must serve a formal Party Wall Notice to affected neighbours, giving them a statutory notice period (usually one or two months).

  • If they consent: The work can proceed.

  • If they dissent (or don’t reply): You will need to appoint a Party Wall Surveyor (or two, if they wish to appoint their own) to draw up a Party Wall Award. This document sets out how the work should proceed, when, and includes a Schedule of Condition for the neighbour’s property to record its state before the work begins.

4. 💰 Budgeting and Financial Planning

Be realistic about the costs. A good rule of thumb is to factor in a 10-15% contingency on top of your quotes, as unforeseen issues (like digging up unexpected drain runs or finding poor ground conditions) almost always arise.

• Quotes: Always get at least three detailed, itemised quotes from reputable builders. Ask to see examples of their previous work and speak to past clients.

5. 🏗️ The Build Team

The success of your project hinges on the quality of your build team.

• Architect/Architectural Designer: Essential for drawing up plans, managing the planning application, and advising on design.

• Structural Engineer: Crucial for calculating and designing the steel beams (RSJs) and foundations that will hold up your new structure and any altered existing walls.

• Builder/Main Contractor: The company that manages the day-to-day construction. Ensure they have appropriate Public Liability Insurance.

6. 💡 Design Considerations

Think beyond just the footprint. How will the new space connect to the old?

• Light: Rear extensions, especially large ones, can make the original part of the house dark. Consider roof lights (Velux windows), large sliding or bi-fold doors, and strategically placed windows to maximise natural light.

• Heating and Insulation: The extension must meet current insulation standards. Think about underfloor heating, which is highly efficient and eliminates the need for radiators.

• Drainage: You may need to divert or protect existing manholes and drainage runs that fall within the extension footprint. This is often an extra cost and requires water authority approval.

✅ Your Next Steps

Before picking up the phone to a builder, start with the planning.

1. Check PD Rights: Use the government’s online planning portal or your local council’s website to check the constraints for your area.

2. Speak to an Architect: Get professional design advice and a preliminary idea of the feasibility and cost.

3. Talk to Neighbours: While not legally required yet, having a friendly conversation with neighbours early on can prevent future disputes, particularly regarding Party Walls.