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A 3 m deep basement has a two story timber frame house above.

They share a side, however both have parts "outside" of the other, as can be seen in the sketch below. The red dash is the basement, the blue is the house.

rough diagram

Can a basement form part of the foundations for the house in conjunction with other foundations not part of the basement?

Are there any risks or known issues with this concept?

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    $\begingroup$ Can you edit your question adding a sketch showing (roughly to scale) what the actual layout looks like? The answer will depend on whether the basement will serve as the foundation for 10% or 90% of the structure above, as well as how they are oriented with each other (is the basement somewhat centered, or is it mostly offset). $\endgroup$
    – Wasabi
    Commented Aug 3, 2018 at 12:06
  • $\begingroup$ Oh, also, what's the type of soil and how deep is the basement? $\endgroup$
    – Wasabi
    Commented Aug 3, 2018 at 14:09
  • $\begingroup$ Cantilever City! $\endgroup$
    – Carl Witthoft
    Commented Aug 3, 2018 at 14:32
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    $\begingroup$ @CarlWitthoft This goes way beyond what DIY would be geared to handle. $\endgroup$
    – user16
    Commented Aug 4, 2018 at 0:12

2 Answers 2

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There are some problems in your sketch that can be fixed. Other than that there is no fundamental issues.

A house or structure can be fully or partially supported by the walls of a basement, provided the bearing walls of the house are supported by bearing walls in the basement and the parts of the house that are outside of the perimeter of the basement have their own foundation.

In your sketch you have not shown the houses bearing walls, but you have to make sure all bearing walls especially the house's exterior walls are directly supported by the walls in the basement. Of course you can have interruptions in those walls such as doors or partial open wall but they have to be engineered.

In most of the US. these are called vertical discontinuities and there are special "penalty" load factors that apply to the structure, sometimes as large as 300% over load.

If it is not practical to have bearing walls under all the house walls in the basement You can have a solid structural concrete slab that runs under the entire house and also covers the roof of basement where it projects out of the foot print of the house, designed to carry the load of the walls. This is actually structurally the preferred method.

The other Issue of concern is the detailed design of decking, roof of basement, water proofing, slope of it and practical flashing, specially at the joints of the house with the roof of basement.

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  • $\begingroup$ Thanks kamran, great information. One further query. What do you mean by decking? I'm not US based and for me it refees $\endgroup$
    – user195166
    Commented Aug 4, 2018 at 7:43
  • $\begingroup$ Thanks kamran. Great information there. What do you mean by decking? I'm Ireland based and I'm not sure what you're referring to. $\endgroup$
    – user195166
    Commented Aug 4, 2018 at 7:52
  • $\begingroup$ By decking I mean the flat roof areas of the basement that project beyond the perimeter of the house. $\endgroup$
    – kamran
    Commented Aug 4, 2018 at 13:20
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Are there any risks or known issues with this concept?

There is excess overhang on the left side, but that leaves plenty of room to bore three holes and fill them with concrete; thus fully supporting that side.

Design 1 - Three Pillars

Another technique is to invert your plans, centering the floors over the basement.

Design 2 - Inverted Plans

The drawing would be better if it were not done on a mobile phone APP.

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