[AdamsLT]

So the idea is to use already existing garages in place of mapping new ones underground, playing with virtual worlds, interiors, etc.

This is not a showcase of a perfect map but an idea of how garages could be used to seem pretty cool "natural" in the GTA:SA landscape.
Madd Doggs mansion has a 3 door garage that can be utilised and are often not.

• Remove the mansion object
• Add the object back
• Add an empty texture to the garage doors
• Add your own doors with the same texture in place of the old ones
• Make them moveable for the owner of the property
• Map the inside of the garage
• Script a teleport inside the mapped garage with a button press for the owner
• Do the same to exit the garage (I recommend letting everyone exit, so as not to have people trapped inside)

Bonus brownie points: add a gate to enter the property with the same texture as a fence and make it seem even neater. Keeping track of the open/closed states of the garage doors would allow you to prevent teleporting inside unless they are open, also would allow people that are not the owner to use the garage in interesting ways.

Note: not all garages can be used like this - some have messed up collisions and that will render the garage impossible to use unless you change the entire house to a working one.

"Why can't you just walk in/out? You can see the outside/inside from the other side!" : The collision is still there, the object just has an invisible texture.