Foundation work for extension on existing buildings

Hello everyone,

We are currently planning the extension and conversion of our small detached house (built in 1938). For this, I need advice regarding a possibly necessary underpinning.

The existing house has one and a half stories and is partially basemented. The existing (formerly coal) basement has a clear height of 1.85 m. The house plinth at the basement is at a depth of about 2.10 m. The non-basemented part of the building has a strip foundation at a depth of about 80 cm.

The extension is to be built on the southern gable side of the house measuring 10x5 m and would be positioned both on the basemented and the non-basemented parts. Now my questions:

- If the extension is not basemented, is underpinning of the existing building still necessary if the extension is built with a strip foundation or slab?
- If the extension is basemented (with a foundation sole of about 2.40 m), underpinning is in any case necessary according to my understanding. If this is built in masonry or concrete, it must be done in sections. Are there current cost indications (per linear meter) for this? How much would the construction of the underpinning extend the build time? Are there any experience-based estimates?

Thank you very much!

A knowledgeable expert (e.g., structural engineer) should look at this. Only they can tell you how your underpinning can be carried out safely.

You probably do not need underpinning, as the old cellars and houses were often built very solidly and can therefore bear the loads of a strip foundation or the slab of the annex.

With the information provided here, the question cannot be answered with certainty.

We do not know your existing floor plan or your property topography. A house built in 1938 will have a 38 cm thick masonry or 40 cm thick concrete-surrounded basement, and usually a cast-in-place concrete ceiling.

Thank you for your quick responses! It is especially important to me that the construction time is not extended by several weeks or months due to the creation of underpinning on the existing building. The gable side of the existing house is about 9m long, which means in my opinion it would have to be underpinned in four steps. With concrete curing times, that adds up. We do not rely on a basement for the annex because of the existing basement, but it is a nice-to-have in case the entire gable side must be underpinned anyway, excavation takes place, and therefore the construction of a basement does not represent a major additional effort.

A house built in 1938 will have a 38 cm thick masonry or 40 cm thick concrete-surrounded basement, and usually an in-situ concrete ceiling.

Correct, 38 cm exterior walls (including basement walls and strip foundation of the existing building without a basement). The plot is flat (i.e. no slope) and the soil is sandy. Groundwater from 6m depth.

You probably don’t need underpinning because the old basements and houses were often built very solidly and can therefore support the loads of a strip foundation or slab of the annex.

How would the connection between the annex foundation and existing foundation be made without a basement under the annex? Would you step down the foundations under the existing house's corners and underpin only the corners as an "extension" of the annex foundation?

Correct, 38 cm external walls (including basement walls and strip foundation of the non-basement existing building).

Why including the basement - I would have expected only the basement - since it is supposed to be a one-and-a-half-story house. At that time, 25 cm would still have been common in the ground floor and upper floor/attic.

For us, the exterior walls on the ground floor as well as in the basement are each 38 cm brick walls.