Step-by-Step Guide to Building Your Home Extension

Building a residential extension is an exciting yet complex process. To minimize disruption and maintain security, many homeowners prefer to knock through into the existing house as the final stage. This approach allows the main house to remain intact and habitable for as long as possible. Below is a structured, step-by-step breakdown of the construction phase, ensuring efficiency and quality throughout.

Site Preparation and Foundations

Before construction begins, the site must be cleared of any obstacles, vegetation, or existing structures that might be in the way. Setting out the extension’s footprint is crucial at this stage, using string lines and pegs to mark exact positions.

Once the layout is confirmed, excavation can begin for the foundations. Depending on the design and ground conditions, this might involve standard trench foundations, raft foundations, or even piled foundations in more challenging sites. Concrete is then poured, providing a solid base for the new structure. It is essential to allow proper curing time before any further load is applied.

Groundworks and Drainage

With the foundations in place, attention turns to underground drainage. Any necessary adjustments or new connections to the existing foul and surface water systems are completed at this point. A damp-proof membrane is then installed to prevent moisture from rising into the new extension. Insulation and a sub-base are also laid before the concrete floor slab is poured and levelled. If required, service ducting for electrical and plumbing installations is incorporated at this stage to avoid future disruption.

Superstructure Construction

The walls of the extension begin to rise, whether built with traditional brick and block, timber frame, or another method. Load-bearing elements such as lintels and steel beams are installed as necessary to support future openings. At this stage, insulation is integrated into cavity walls, and fire and acoustic barriers are positioned according to regulations.

As the walls take shape, the roof structure is erected, whether using traditional rafters, trusses, or a flat roof system. A breathable membrane is laid, followed by the final roofing materials such as tiles, slates, or a flat roof covering. Guttering, soffits, and fascias complete the exterior of the roof, ensuring proper water drainage and a polished look.

Windows, Doors, and External Finishes

Once the structure is watertight, external windows and doors are installed, further protecting the interior from weather exposure. Depending on the design, external finishes such as brickwork pointing, rendering, or cladding are applied. Any additional structural elements like porches, canopies, or decorative features are also added at this stage.

First Fix Services

Before plastering can begin, first fix services must be installed. This includes running electrical wiring, plumbing pipework, and ventilation ducting through walls, floors, and ceilings. Electrical back boxes are fitted, and any necessary openings for radiators, underfloor heating, or ventilation systems are prepared. With these in place, final structural checks are conducted to ensure all lintels, beams, and supports are correctly positioned and secured.

Internal Finishes

The interior walls and ceilings are now ready for plasterboard installation. Once the boards are in place, plastering begins, providing a smooth, finished surface. The drying process must be carefully managed to prevent cracking and ensure an even result.

With the walls complete, attention turns to the flooring. Depending on the build, this may involve installing additional insulation, laying a screed, or fitting floorboards. Any necessary levelling is done before the final floor covering, such as tiles, carpet, or wood, is installed.

Second Fix Services and Fixtures

Now that the internal surfaces are prepared, second fix services can be completed. This involves installing electrical sockets, light fixtures, switches, and plumbing fittings such as taps, toilets, and sinks. Heating systems, whether radiators, underfloor heating, or alternative methods, are connected and tested.

Joinery elements such as internal doors, skirting boards, and architraves are fitted at this stage, adding to the finished aesthetic. Built-in storage or bespoke carpentry features are also incorporated as needed.

Knocking Through to the Existing House

Only once the extension is fully built, weather-tight, and internally complete does the process of knocking through to the original house begin. This ensures that disruption, dust, and security concerns are minimized throughout the build.

Before the opening is made, structural preparations are essential. Temporary supports may be needed to prevent any movement or stress on the existing building. When ready, the wall section is carefully removed, creating the new access between the original house and the extension. Any necessary lintels or steel beams are installed to maintain structural integrity.

Once the opening is made, the connection between the old and new spaces is carefully finished. This includes patching up plaster, ensuring airtightness, and making any necessary adjustments to insulation and damp-proofing layers. The transition between spaces should be seamless, both visually and in terms of energy efficiency.

Final Finishing and Handover

With the structure now fully integrated, final finishing touches bring the project to completion. Decoration begins with painting or wallpapering, followed by the installation of final floor coverings if they were not completed earlier.

Before handing over the completed extension, a thorough snagging process takes place. This involves checking for any minor defects, testing all services, and ensuring compliance with Building Control regulations. Electrical and plumbing systems undergo their final inspections, and any necessary adjustments or fixes are made.

Once everything is approved and signed off, the protective coverings are removed, and the space is given a final clean. The extension is now ready to be used, seamlessly integrating with the existing home while providing the additional space and functionality required.

By following this structured approach—knocking through last—homeowners can maintain a secure, livable home for most of the project while ensuring a high-quality build with minimal disruption.