Extending an existing home: what should you consider?

Published: 1 November 2026.

An extension creates more living space, but the new structure must connect well to what is already there. The existing structure, foundations, building services and finishes influence the right solution. Investigate these points early so that the design, budget and construction plan share the same assumptions.

Start with the existing building

Record how the home is built and which parts you want to keep. Floor plans and old construction drawings help, but may not reflect the current situation. A survey of the rear wall, floor, crawl space and services shows which questions still need investigation. Check access to the garden and the space available for materials and equipment.

Creating an opening in the rear wall

To connect the new room openly to the home, part of the existing rear wall often has to change. The width of the opening is only one consideration. Loads from floors and the roof above, existing beam positions and temporary support during the work also matter. Have the structural solution developed before finalising the glazing and interior layout.

Foundations and potential movement

The extension foundation must suit the ground and the loads from the design. The new and existing parts of a house may behave differently. Discuss the junction, any movement joints and the implications for walls and finishes with the designer and structural engineer. If the existing foundation is uncertain, further investigation may be needed.

Insulation, ventilation and comfort

A larger living space changes how you heat and ventilate the home. Pay attention to the junction between old and new floors, walls and roofs: poorly designed details can cause thermal bridges, draughts or moisture problems. Discuss ventilation, shading and lighting, and whether existing building services need to be altered.

Weatherproof details and a coherent finish

The junction of the new roof and walls with the existing home needs particular attention. Consider rainwater drainage, thresholds, windows and the transition between old and new floor finishes. A tidy drawing does not show every detail behind the finish. Ask which junctions will be designed and what is included in the quotation.

Permits and scheduling

Whether you need a permit or notification depends on the proposal and its location. The Dutch government’s Environment Information Point advises homeowners to use the Permit Check and contact their municipality if anything is unclear. Allow for that outcome and any necessary surveys in your schedule. Also agree which rooms can remain in use during construction and when water or power may need to be turned off temporarily.