errhm, we do the same, and rent the place in the garage separately to the flat,and it's also clearly stated in our renting contracts.

If someone uses only public transport and has no need they then do NOT need to rent a place,but for those who got a car we want them to take one as we have only three visitors places outside and no desire to have them clogged up all the time by ppl who live in our house and want to save somebucks on the back of the others,the third option is, if they got a car and don't want to rent a place, then they have to look elsewhere to park their car.

This is only fair towards all the other tenants, who pay for their place!

So it's worth checking out the details why they want you to rent a place in a garage/parking space as well.