The most expensive part of a house extension is typically the structural work, including foundations, roofing, and load-bearing walls. If major excavation or underpinning is required, costs can escalate quickly. Adding a second storey is particularly costly due to the extra engineering requirements and reinforcement needed to support the additional weight.

Other costly elements include:

  • Plumbing and Electrical Work: If you’re adding bathrooms or kitchens, rerouting plumbing and wiring can be expensive.

  • Custom Materials and Finishes: High-end materials, custom joinery, and luxury finishes drive up costs.

  • Windows and Doors: Large glass panels, bi-fold doors, and skylights enhance aesthetics but can add substantial expenses.

Keeping costs under control involves careful planning, selecting cost-effective materials, and working with experienced professionals to avoid costly errors.