ᐅ Extension of a Single-Family Home to an Existing House – Stepped Foundation

Hello,

the plan is to build an additional single-family house (about 8.50 x 7.50 m (28 x 25 feet)) attached to the existing house (about 8.50 x 10 m (28 x 33 feet)).

The old house has a basement; we want to build without a basement using a timber frame construction.

As far as I know, the foundation must be excavated down to the depth of the old house’s basement, and a stepped foundation is required.

What are the approximate additional costs compared to a standard slab-on-grade foundation? Are we talking about around 1,000 or 10,000 euros? Is it even possible to estimate roughly?

Thanks and best regards

Kat097 schrieb:
As far as I know, you have to dig down to the depth of the old house’s basement and create stepped footings.

From your description, I don’t see any need for that. It's common to build a garage without a basement directly next to a building that has a basement. Attached houses, one with a basement and one without, are generally not a problem as long as the one without a basement is the one built later.

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The foundation must be established on undisturbed soil. Immediately adjacent to the basement, there is always the excavation pit with backfilled soil, which is usually not compacted to the same standard as undisturbed soil. This must be verified using appropriate methods, such as a dynamic probing test. If necessary, the foundation must be placed on the top level of the undisturbed soil, for example, using lean concrete up to 80cm (31 inches) below the finished ground level at frost-free depth. In other locations, this depth can be reached without deep foundation work. Deciding this is the responsibility of the designer or structural engineer. Obtaining this information is the homeowner’s responsibility. The additional costs cannot be specified without knowing the exact circumstances; however, a realistic estimate would be between €1,000 and €10,000.