ᐅ Is an existing 30 cm foundation strong enough to support an additional story?

Hello everyone,

We would like to add an additional story to our old, single-story house with an attic, using a timber frame construction. Since only a few documents exist, the foundation was partially exposed in some places to assess its condition.
The house measures 12m x 6.5m (39.4 ft x 21.3 ft) and has a continuous strip foundation about 28–30cm wide (11–12 inches) and 75cm deep (29.5 inches). Above it, the walls are made of 24–30cm (9.4–12 inches) thick sand-lime brick. Additionally, in the middle of the longer side, there is a foundation strip 30cm wide (12 inches) with a 24cm (9.4 inches) thick sand-lime brick wall. According to the soil report, the ground is supposed to have good load-bearing capacity.
Our structural engineer mentioned that the foundations might be too narrow. (Calculations are still pending.)
Is this really tight in terms of sizing? How could the foundation be reinforced? Simply adding another strip next to it seems too simple.

Best regards,
Werner

The width of the foundation is less important. The key factors are the depth, overall load-bearing capacity, and the soil conditions.

Since my personal opinion is completely irrelevant here, I leave such matters to the structural engineer. They are the expert in this field.

Thanks in advance. I just want to get an idea of what to expect as much as possible.

Werner0 schrieb:

The structural engineer mentioned that the foundations might be too narrow. (Calculation is still pending)
Is it really designed that tightly? How could the foundation be reinforced? Just adding another strip next to it would probably be too simple.

Hello,

this option is definitely possible. However, it’s not as simple as just casting concrete onto the existing foundation.
Reinforcement bars can be bonded into the existing foundation using epoxy injection mortar, and the new longitudinal reinforcement of the foundation widening can be tied to them. It is feasible, though it requires effort.

This approach is not uncommon in industrial building construction when foundations need to be enlarged for hall extensions.