We have an attached garage. The entrance to the garage is in an interior hallway. I'd like to break the wall a bit & install interior upper cabinets and countertops on the inside of the hallway. The garage is much lower than the inside hallway and cars in the garage (on the other side of the wall) would NOT lose any parking area/space. I'd like to gain some cabinet storage inside our house. Is this a terrible or doable idea? If so, what are the main issues to watch out for? Thanks!!
It all depends on the wall. An attached garage wall is an important firewall, with requirements far stricter than most other areas of the house. Actually, most of a house (with the exception of bathrooms) could be one big open space except for the garage.
This link has a good writeup based on the 2006 IRC. As I understand it, unless you have something stronger (e.g., a brick or cinderblock wall (common in older construction or where the garage was added after the house was built, but far less likely in new construction because it costs more), you need to have 1/2" drywall - with joints, edges, holes all sealed very well - on the garage wall as a firewall. There are also requirements for doors - your ordinary interior door won't do even though from the perspective of residents it is an interior door.
End result: You need to see what is inside your walls before knocking them down. If you have brick or block on the garage side then you can likely tear down drywall and (provided they are not load bearing, or with appropriate changes to take care of loads) even remove some studs in order to get more space for cabinets. If you have an ordinary wall with drywall on both sides and studs in between, you should be able to remove the inner drywall, but you need to be certain that the garage drywall remains intact (and/or is patched as needed) so that you still have a proper firewall. On the other hand, if you have drywall on the inside and open on the garage side (i.e., inside the garage you currently see studs and the back on the hallway drywall) then the inside wall is the firestop and you can't do anything to that wall without adding drywall on the garage side - with extreme attention to joints, edges and other gaps. As I understand the IRC, there should never be "drywall only on the house side" but I have seen that - maybe not to code, or maybe the code has changed.
You may also have an issue, particularly if you can't borrow space from the wall cavity, of hallway width. Hallways should normally be at least 3 feet wide. While there is likely some leeway for an unnecessary hallway - e.g., a hallway that leads only to a storage area - any hallway that goes between living areas and a functional entrance (which certainly includes a garage) should meet that minimum. So if your hallway is 4' wide that gives you 1' to play width. If it is less than 4' wide then this may not be doable at all.