Often properties are extended above an existing garage. One option is to demolish the existing garage, dig out the foundations and start again. If the garage has poor foundations, this is normally the best, and sometimes the only solution. However, if the garage was built relatively recently and has good foundations (or even if it is an old garage with good foundations) there is normally no reason why another storey can't be added. This can result in a significant time and cost saving.

The main structural issue is that the walls of the extension are likely to be much thicker than the existing garage walls. A new cavity wall will be around 300mm wide, whereas garage walls were generally built as 100mm thick walls with piers. It is possible to overcome this by providing steel beams (normally 3 beams are needed) at first floor level to support the extension on the existing garage walls.

What constitutes a good foundation? If the existing foundations are at least 450mm wide and at a depth of at least 750mm below ground level, assuming the ground is reasonably good, then the foundations are likely to be adequate. However, if the founding material is a shrinkable clay and there are trees close to the building, the existing foundations may not meet current regulations either with or without the extension. Your local Building Inspector will advise you on the adequacy of the ground and the existing foundations.

Our structural calculations for an extension above an existing garage includes the design of any necessary steel connections and padstones. Our structural design also includes checking the load bearing capacity of the existing garage wall.

We need the following information to design an extension over an existing garage:

architectural layouts and elevations, or sketch drawings with dimensions

details of the existing garage walls and the foundation construction

Design information

All design work is carried out in accordance with BS5950 (The structural use of steelwork) and BS5628 (The structural use of masonry).

The calculations provided will be suitable for Building Regulation approval.

We recommend that you satisfy yourself that the existing foundations are adequate before asking us to carry out our design work. The best way to do this is for you to excavate trial holes to expose the existing foundations and the ground beneath them, and to let the Building Inspector have a look.