Extending at the back of the house without settlement: do you need a separate structure? The answer and the right way to do it

A common question for people planning a back kitchen or laundry area is "Does a back extension need to be structurally separate from the main house?" The short but firm answer is "absolutely, yes" , because the soil and pile loads of the main house and the extension are completely different. Tying the structures together is the number-one cause of cracked walls and leaking roofs.

Why do they need to be separate? Because the main house usually sits on piles driven deep into the hard soil layer (e.g. 15-20 m driven piles), so it settles very little, while back extensions usually use smaller piles and settle faster. If the extension's structure is attached to the main house, then as the back part sinks it will "pull" on the walls and roof of the main house, causing cracks.

A 4-technique checklist for a proper back extension that prevents cracking and settlement

For a back extension that is safe and lasts, follow these principles for separating the structure:

• 1. Leave an expansion joint: leave a gap of about 1-2 centimetres between the old and new structures so walls and beams don't touch directly
• 2. Choose the right piles: if your budget allows, we recommend "micro piles" , which can be driven down to the hard soil layer as deep as the main house's piles, reducing settlement most effectively
• 3. Seal the joint with a flexible material: push backer rod into the joint, then seal over it with polyurethane sealant (PU sealant) to keep water out while still letting the structures move
• 4. Roof it separately: don't drill and fix the new roof frame directly into the existing house wall; give it its own posts, and use flashing tucked under the existing eaves to keep rain out

Important caution: Separating the structures does a good job of reducing cracks, but extensions involving the building structure, loads and electrical systems should be advised on and designed by a structural engineer and specialist tradespeople only, to calculate the loads correctly to engineering standards and comply safely with the building setback law.