Should the garage yard be supported because of the basement floor?

Good evening,

I am new here and two days ago we received our building permit. We managed to secure a “last” plot for a “terraced end house” which, according to building specifications, must be built wall-to-wall with the existing buildings. On the left a fully basement house, on the right a garage courtyard (not basemented!). The garage courtyard consists of 10 garages each 3m wide, we are building 18m onto the garages and of course want a full basement. (Currently unknown how the garages are constructed). The architect once estimated 10,000 EUR for the supports, but I wonder: a) is this cost estimate realistic and b) how is the whole thing supposed to be done? The garages don’t belong to us and if they settle or cracks appear just when digging... that already makes me uneasy. Experience reports or general info on this topic from experts would be great! Thank you very much M.C.

Just ask around in the neighborhood, there are dozens of end-of-terrace houses in the same situation, so garages built onto them.

there are dozens of end-of-terrace houses in the same situation, that is, built onto such garages.

Dozens? - well, I do see several end-of-terrace houses directly on garages (in the aerial view in wehwehweh); but I cannot tell from my crystal ball which a) are younger than the garages and b) have basements. The problem basically only occurs if the house is built after the garages and has a basement.

(Currently unknown how the garages were constructed)

That looks like concrete row prefabricated garages to me, which typically stand on concrete sleepers / strip foundations.

Hello Cadenama

The garages definitely need to be underpinned. Not a big problem. It’s just labor-intensive. Because the underpinning has to be done in small sections.
About 150 cm in depth from the future basement down under the garages will be excavated and filled with concrete. And this will be done until the garages are underpinned along the entire length. You just have to get a cost estimate. The 10 thousand euros are quite realistic.
A little tip: go with the architect (as a witness) into each garage and look closely at the walls. Take photos. Otherwise, after the construction work, existing settlement cracks will gladly be blamed on you. And if settlement cracks do actually occur, you can immediately pass the blame on to the construction company.

Steven