Can You Build the Flank Wall of a 6m Rear Extension Over the Boundary?

Can You Build the Flank Wall of a 6m Rear Extension Over the Boundary?

Building a 6 metre rear extension under prior approval is an excellent way to maximise your habitable space on the ground floor of the house without the need for full planning permission. However, questions often arise about property boundaries and shared walls, particularly when it comes to the flank wall (another word for "Side wall") of the extension.

A common query is whether you can build this wall 50% over the boundary line, so your neighbour can share it in the future. While this might seem practical, it could conflict with the permitted development (PD) rights that allowed the extension to be built in the first place. Let’s unpack this scenario to determine the best course of action.

Understanding Prior Approval and Permitted Development Rights

A 6-metre rear extension falls under Class A of the General Permitted Development Order (GPDO). While prior approval through the neighbour consultation scheme is required for larger extensions (beyond 3 metres for terraced and semi-detached homes, or 4 metres for detached homes), the process does not negate the fundamental principles of PD rights.

One of the key requirements for PD rights is that the development must occur entirely within the boundary of the property. This means that any part of the extension crossing into your neighbour’s land—such as the flank wall or foundations—could invalidate the original prior approval.

Can You Build the Flank Wall Over the Boundary?

Shared Walls and the Party Wall Act

The idea of building the flank wall partly over the boundary may appeal to homeowners and neighbours hoping to share construction costs or reuse the wall for future extensions. However, this introduces several complications:

1. Conflict with Permitted Development Rules:
   • PD rights stipulate that the development must stay within your property’s boundary. Crossing the boundary even by a small amount could invalidate the prior approval.
   • This situation could result in the local council declaring the development unlawful and requiring a retrospective planning application or appeal against enforcement action and in worst case scenario ordering the demolition of the extension which was built unlawfully.
2. Party Wall Implications:
   • If you intend to build the flank wall as a party wall, the Party Wall Act 1996 requires formal notification to your neighbour. This is a separate legal process and does not automatically grant you permission to encroach on their land. If you require assistance from a Partywall Surveyor, let us know and we can recommend one.
3. Ownership Disputes:
   • Building over the boundary without an agreement could lead to disputes, and your neighbour may not grant retrospective consent. This could create legal complications or result in costly alterations.

Best Practices to Avoid Boundary Issues

To ensure your 6 metre rear extension complies with prior approval requirements and avoids future disputes, follow these best practices:

1. Keep the Extension Fully Within Your Boundary

The simplest solution is to ensure the extension’s flank wall and foundations are entirely within your property boundary. This eliminates any potential for conflict with permitted development rules or boundary disputes.

2. Document the Start of Construction

To protect yourself, take photographic evidence of the site at the commencement of works. This should include images of:

• The trenches dug for the foundations.
• Measurements showing the trenches are entirely within your property boundary.

This evidence will help prove later on that the extension commenced lawfully under the original prior approval.

3. Discuss the Party Wall Act Early

If you and your neighbour wish to share a future wall, consult a party wall surveyor before construction begins. This ensures compliance with the Party Wall Act and helps establish clear agreements about costs and use.

Risks of Building Over the Boundary

If you proceed with building the flank wall over the boundary without proper permissions, several risks could arise:

1. Invalidation of Prior Approval:
   • If the local council discovers the encroachment, they may revoke the permitted development status and require retrospective planning permission.
   • This could delay your project and result in additional costs if the planning application is refused such as the need to appeal against enforcement action or other potential legal issues.
2. Enforcement Action:
   • A neighbour unhappy with the encroachment could report the development, leading to enforcement action by the council.
3. Future Complications:
   • When selling your property, any irregularities in the extension’s approval process could create difficulties in conveyancing and delay the sale process or reduce the property’s value.

Conclusion: Avoid Boundary Encroachment

While sharing a flank wall might seem practical and the right thing to do, building it over the boundary could undermine the original prior approval and result in complications with PD rights. To safeguard your project:

• Ensure the extension remains fully within your boundary.
• Keep photographic evidence of the foundation trenches to prove lawful commencement.
• Consult a party wall surveyor if shared wall use is a future goal.

By taking these steps, you’ll avoid costly disputes, ensure compliance with permitted development rules, and complete your extension with confidence.

If you wish to discuss your project with our team, please give us a call and we will be happy to advise on the best approach and if necessary, help with the architectural drawings for Building Control (building regulations) and for the Partywall Agreement.