Structural Requirements for Extensions and Alterations

What This Requirement Covers

Building an extension or altering an existing building requires careful consideration of the structural impact on both the new and existing construction. Approved Document A requirements apply to the new work, and Regulation 6 requires that the existing building is not made worse by the alteration. Structural design for extensions is typically carried out by a structural engineer working to the Eurocodes.

Key Requirements

Foundations

1. The extension foundations must be designed independently of the existing building foundations
2. Where the extension abuts the existing building, differential settlement must be considered
3. Foundation depth must account for existing drainage runs, trees, and ground conditions
4. If the extension foundation is deeper than the existing building foundation, precautions must be taken to avoid undermining the existing structure

Connection to Existing Building

1. The junction between the extension and the existing building is a critical detail
2. A movement joint is often provided to allow for differential settlement between the existing and new structures
3. Where a rigid connection is required, the existing wall may need to be tied into the new structure with appropriate fixings
4. Existing wall ties, lintels, and foundations should be assessed for adequacy if additional loads are being imposed

Structural Openings

1. Creating an opening in an existing wall to connect the extension to the house requires a structural assessment
2. A steel beam (RSJ) or concrete lintel must be designed to carry the loads above the opening
3. The beam must have adequate bearing on each side (minimum 150 mm on masonry, or as specified by the engineer)
4. Temporary propping is required during the works to support the structure above while the opening is formed and the beam installed
5. A Building Regulations application is required for structural openings

Removing Load-Bearing Walls

1. Internal load-bearing walls can only be removed or altered with a structural engineer's design
2. A steel beam or equivalent must be installed to carry the loads previously supported by the wall
3. Padstones or bearing plates may be required to spread the beam reactions
4. Building Control approval is required before removing any load-bearing wall

Roof Connections

1. Where an extension roof meets the existing roof or wall, the junction must be weatherproofed and structurally sound
2. Lead flashing or proprietary cavity trays must be used at abutments
3. Additional roof loads from the extension must not overload the existing wall

Practical Compliance Tips

1. Appoint a structural engineer at the design stage, not after Building Control raises queries
2. Obtain a structural survey of the existing building before designing the extension, especially for older properties
3. Do not assume existing foundations are adequate for additional loads; investigate their depth and type
4. Plan the temporary works (propping, shoring) carefully for structural openings; inadequate propping can cause cracking or collapse
5. Submit structural calculations to Building Control with the full plans application
6. Photograph the temporary works and beam installation for the building file
7. Allow for movement at the junction between the extension and the existing building to prevent cracking