Staalgoed Handel B.V.

Foundation for an annex to a house, what do you need?

20-02-2018 | J.van Wijngaarden

If you want to extend your existing home, well begun is half done. Therefore, ensure a correct foundation.

In the first place it is recommended that the strength of the desired annex to a house is calculated by a constructor. With this calculation and drawing, which includes the reinforcement, you can can started.

Formwork for the foundation

After the necessary digging and possible pile drinving, which is a result of the construction report, it is possible to start with the formwork. EPS formwork is often chosen for an annex to a house. This  type of formwork is easy to transport and simple and quick to install.

Fundering voor een aanbouw

Reinforcement, or rebar

The width and height of the rebar cages depends on the required foundation beam. The most common foundation beam sections for an annex to a house are:

  • Small annex to a house: 300 x 400 mm concrete beam with 3 x diameter 10 mm bottom and top reinforcement.
  • Medium-sized annex to a house: 350 x 400 mm concerte beam with 3 x diameter 12 mm bottom and tope reinforcement.
  • Large annex to a house: 350 x 500 mm concrete beam with 3 x diameter 12 mm bottom and top reinforcement.

The reinforcement for the foundation beams, alias rebar cages, is usually available in standard cages with the dimensions of 3, 4 or 6 meters . Nowadays, customization can also be realized simply and well through the latest technology.

Connecting the rebar cages is possible with lapping bars and you can easily slide the corners together using different ends of the cages.

To ensure that the reinforcing bar remains in place during the pouring of concrete, the individual bars or prefab elements are connected to each other with tying wire.

For the slab reinforcement of the foundation reinforcing mesh are used. The requested reinforcement for a floor is often ø6mm or ø8mm with a wire spacing of 100x100 or 150x150.

Depending on the thickness of the floor, 1 layer or 2 layers of reinforcing mesh are applied. Floors with a floor zone up to approx. 120 mm are often with 1 layer of reinforcing mesh. Thicker floors from around 150 mm are usually provided with 2 layers of reinforcing mesh.

Lattice girders are used with 2 layers of reinforcing mesh to get the reinforcing mesh to the correct height.

Bouwstaalmatten voor fundering


Spacers are used to prevent decay of concrete. These spacers ensure that the reinforcing steel is properly enclosed by the concrete.

Often 35mm is chosen for this, for the cages as well as for the slab reinforcement.

Plastic ring spacers or flat base spacers are used on the sides of the rebar cages. The guideline for the correct number is 6 spacers per 1000 mm rebar cage. Underneath the rebar cages, concrete spacers are being placed. The guideline for the correct number of concrete spacers is 3 spacers per 1000 mm cage.

Concrete spacer bars or concrete spacers can be used for the correct coverageage of the slab reinforcement. The guideline for this is 2,5 spacer bars per 1 m2 or 3 spacers per 1 m2.


When all the reinforcement has been applied and checked, it is time to pour the concrete. Depending on the required quantity, you can make this yourself, pick it up at a concrete station or have it delivered by a concrete plant.

If, after reading this article, you still have questions about creating a foundation, please feel free to contact us. We are happy to help you further!

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Do you actually find ordering reinforcement yourself quite a chore?

We can also work out the reinforcement for you!

You will then get:

  • A 3D model of the reinforcement, viewable with the RebarAssist.
  • Clear drawings of all prefabricated parts.
  • Detailed elaboration of the reinforcement.
  • Assembly plan of the reinforcement.
  • Clear and numbered parts list, corresponding to the assembly plan and labels to the reinforcement.
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Foundation for an annex to a house, what do you need? - Do you actually find ordering reinforcement yourself quite a chore?