I own a 1980, Bruce Brownell passive solar home in the Adirondacks.
It has an attached single car garage with an attic and foyer that are on a concrete slab.
The problem is my house is on a hill and the driveway is too steep in winter.
I was wondering if you think it would be possible to design a "lower two or three car garage" underneath the current slab.
If I could, I could convert the garage/foyer/attic into more house space or a sun room.
Or, do you think I have to take the addition off and start from scratch?
Thanks for any advice.
In my opinion, it is highly unlikely that you could construct new space under an existing garage slab.
Most slabs on grade contain minimal reinforcing and cannot support themselves, not to mention people and furniture, if the underlying soil is removed.
It is also likely that the foundation supporting your garage is not deep enough for an additional story.
Instead of the approach you outlined, I would consider the following:
1. Install temporary supports for your existing garage walls and roof.
2. Remove the existing slab and foundation excavate for the new lower story.
3. Construct the new lower level garage with a wood framed second floor to support your existing walls and roof.
Good luck with your project.
David Moore, AIA
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Note: The basic difference in estimating a home addition vs. a complete home will be the increase of some square footage building costs due to the smaller size of the job.
You might want to read Cost of Home Additions as well as Cost to Build a Home Addition with Garage Underneath.