If you are planning a home extension, one of the first questions you will face is whether you need groundworks before building can start. The short answer is yes — every extension requires proper groundworks. Without them, your builder has nothing solid to build on, and you will not pass building control inspections.
Groundworks cover everything that happens below ground level and at ground level before the walls go up. For an extension, this typically includes site preparation, excavation, foundations, drainage and dealing with any existing underground services.
Before any digging starts, the site needs to be cleared. This means removing turf, paving, plants, sheds or anything else sitting where the extension will go. Once cleared, the ground is excavated to the required depth for the foundations. The depth depends on soil conditions and what building control requires, but trenches for a standard house extension are typically around one metre deep. A mini digger is usually needed for this work, along with a way to remove the spoil from site.
The foundations are the most critical part of the groundworks. For most residential extensions, you will need either strip foundations or trench fill foundations. Strip foundations involve pouring a concrete strip at the bottom of the trench, then building blockwork up to ground level. Trench fill is quicker — the trench is filled almost entirely with concrete. Your structural engineer will specify which type is needed based on the ground conditions and the load the extension will carry.
Extensions often sit over or near existing drainage runs. If a drain runs under your proposed extension, it will need to be diverted or protected with a lintel. New drainage connections may also be required for kitchens, bathrooms or utility rooms being added as part of the build. All drainage work must comply with Building Regulations Part H, and connections to the public sewer usually need approval from your local water authority.
Gas pipes, water mains and electrical cables sometimes run through the area where an extension is planned. These services need to be located, protected or rerouted before excavation begins. A competent groundworks contractor will carry out a services survey as part of the preparation work to avoid any costly or dangerous surprises during the dig.
All groundworks for an extension must comply with Building Regulations. A building control inspector will visit at key stages — typically after the trenches are dug (before concrete is poured) and again once the foundations are complete up to damp-proof course level. If the groundworks are not done to the required standard, building control will not sign them off and your project will stall.
For a standard single-storey rear extension, groundworks typically take one to two weeks. This covers excavation, foundations, below-ground drainage and getting the slab down ready for the bricklayers. Larger or more complex extensions, or sites with difficult ground conditions, may take longer.
Getting the groundworks right is essential. Mistakes at this stage are expensive and difficult to fix once the build is underway. A professional groundworks contractor will have the plant, equipment and experience to get the job done efficiently and to the standard building control expects.
If you are planning an extension and need reliable groundworks, MJM Groundworks can help. We handle everything from excavation and foundations to drainage connections across Surrey, Berkshire, Hertfordshire and London. Get in touch for a free, no-obligation quote or call us on 01784 697317.