Extension foundations: which foundations does your extension need?

The foundations of an extension carry everything above them: floor, walls, roof and glazing. In West-Friesland and the rest of Noord-Holland the subsoil often consists of clay and peat. That makes piled foundations the rule rather than the exception here. On firm sandy soil, shallow strip footings can be enough. Which extension foundations you need depends on the soil, the weight of the new structure and the foundations of your house. A cone penetration test and the advice of a structural engineer give certainty.

Why are the foundations of an extension so important?

An extension connects directly to the existing house. If the extension settles differently from the house, you get cracks at the junction, sticking doors or a leaking roof edge. That is awkward and expensive to repair afterwards.

With an extension we therefore always start below ground. Only once the foundations are settled do we look at the floor, the facade and the roof.

What is the difference between strip footings and piled foundations?

With shallow strip footings, the extension rests on concrete strips in a load-bearing layer just below ground level. That works well on firm sandy soil. In Dutch this is called a foundation op staal, where staal refers not to steel but to the firm ground.

With piled foundations, piles transfer the weight to a deeper, load-bearing sand layer. Concrete ground beams are placed on the piles. On soft ground, as in large parts of Noord-Holland, this is often necessary.

• Strip footings: for firm ground just below the surface
• Driven piles: proven, but with vibration and noise
• Screw or bored piles: little vibration, ideal for a terrace
• Choice always based on soil testing and an engineer

How do you know which foundations your extension needs?

A cone penetration test shows how the soil layers on your plot are built up and at what depth the load-bearing layer lies. A structural engineer uses that data to determine the type of foundation, the pile length and the dimensions.

Also look at the foundations of your house. If the house stands on piles, it is sensible to put the extension on piles too. That way house and extension settle as evenly as possible.

Extension foundations: how do you connect to the existing house?

Between the new foundations and those of the house there is usually a movement joint: a deliberate gap so that both can move independently. In the finish that joint is neatly concealed. This prevents small differences in settlement from causing cracks.

The floor goes on the ground beams, often an insulated precast floor system or a cast concrete floor. At our rear extension in Zaandam the cast floor sits against the existing rear facade, with the insulation boards ready for the next step. A well insulated floor is part of a well insulated extension.

What do the foundations of an extension cost?

The foundations are often a large part of the total building costs, especially if piles are needed. The number of piles, their length, access to the back garden and soil removal determine the price. A back garden without rear access, for example, makes the work more laborious.

For a rough indication of the whole project, see what does an extension cost or extension costs per m2. The foundations themselves can only be priced properly after soil testing and structural advice.

Frequently asked questions