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Short Stages for an Extension Foundation

Short Stages for an Extension Foundation

Setting out

Getting wrong in excavating is all too easy. If you end up having to have extra trenches filled with concrete, this means extra cost for you.

Even when dimensions haven’t been specified, planning enforcement officers can scale off the drawings to check that your home is where it should be. The distances to boundaries and the levels are both important from a planning aspect, because of the issues of overlooking and

Trench fill foundation

Trench fill avoids bricklaying below ground. The concrete is poured to within 150mm of the surface ground level, saving time and trouble. The sides of the trench play as much a part in supporting the load as the bottom, and so this foundation must only be used in stable ground, where the trench sides are firm and capable of bearing loads. Clay and chalk soils are ideal for trench fill foundations.

Strip foundations

Usually wider, strip foundations use less concrete by being thinner. They’re typically 300mm thick. The exact thickness will need to be determined by the masonry courses of the walls up to the damp-proof course. Strip foundations are often necessary for softer soils, such as sand, since they spread the load of the building out over a greater area.

Building control

The building control officer must be notified at a certain stage and await their inspection. The contractor could be assigned for this duty, but check for yourself that the works have been inspected and approved before covering that stage and continuing. Most building control authorities are happy to book inspections from a phone call, email or fax, so long as they are given a day’s notice. The same goes for your insurance warranty inspector, who will also need to inspect at some stages.

Building control inspection schedule for groundwork:

  • Commencing the work Give two days’ notice

  • Foundation excavations Give one day’s notice

  • Over site preparation Give one day’s notice

  • Drains laid in trenches Give one day’s notice

  • Drains testing Give one day’s notice


The easier way is to order a ready-mix lorry that can be placed by the pump, up to a distance of 40m. Pumps must be booked in at an early stage.

Ground-bearing floors

Beneath a ground-bearing concrete floor slab for the extension, the ground must be properly prepared with an oversite treatment, to be stripped clear of topsoil and vegetation before the foundations were dug.

Hardcore should be used in a layer, at least 150mm thick, but no greater than 600mm, in selected aggregate. It’s then compacted down in layers with a plate compactor.

Hardcore rubble needs to be clean, broken brick, concrete or stone that has been crushed or is naturally available in less than 100mm particle sizes.

On sloping sites, where part of the oversite may exceed 600mm deep, it may be acceptable to use lean-mix concrete in layers with the hardcore to make up the difference.

Lap and tape down 1,200 gauge polythene for the damp-proof membrane and make sure it is dressed up over the walls so it can be lapped with the damp-proof course later. The insulation boards can be laid out over the damp-proof membrane and covered by an additional layer of polythene before placing the concrete. These boards should also be cut to an up stand around the external walls to encase the concrete slab fully in a warm tray.

Angular hardcore needs binding with a thin layer of sand to protect the polythene damp-proof membrane from puncturing and also to create a level bed for the insulation to sit on.


Tamping the concrete helps to consolidate the concrete poured for the extension floor.


Drainage tips

  • Lay pipes to an even gradient

  • Lay pipes in straight lines and avoid bends

  • Locate the soil vent pipe at the head of the run

  • Provide rodding points or inspection chambers at bends and changes in gradient

  • Connect water closets and soil vent pipes direct to inspection chambers

  • Use at least 100mm of gravel to cover the pipes before back filling the trench and don’t leave the trench open for longer than is necessary

If you would like to know more about the building of your extension you can contact us any time during working time. We will be happy to assist you with your Extension Building Project from design, building to finishes.

Contact us:

A to Z Design and Build ltd

20 Dawes Road,



tel: 07578930828


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