If the garage is not exactly on the boundary, it triggers a setback area, doesn't it?
In some federal states, you either have to build on the boundary or maintain a minimum distance, e.g. 1m in HE and NI, 0.5m in BW, in others the distance is arbitrary, like with the OP (ST) or with me (NRW). I think the one meter is reasonable though.
To my knowledge, that does not apply to garages.
That is the case in ST, at least if they are not higher than 3m and not longer than 9m.