Could you just use one of your offices/studies as a bedroom for one of your teens? It might mean you'd have to temporarily relocate that space to another area of the house or share one for you and your DH, but this way you don't have to add anything else in. That is assuming that it would be a short term arrangement.

Another option is to add a small guest house if when this happens and it is for the longer term. In the time that would take you could just use the above short term arrangement.

your mother would live in the room(s) over
the garage? think about if it would be
or would become difficult for her to climb
stairs.

most folks I work with that plan future rooms
over garage (frog's) go ahead and do the electrical
plumbing & sheetrock as it is cheaper while these
trade are onsite building main house.
also think about wider doors ..36" for walker
or wheelchair acessability.

these rooms and rooms in attic are often hard to heat and cool. this is because they are essentially located in the hottest/coldest part of the house..the attic.
hot/cold air is on all sides, top and bottom of these areas
so they should be better insulated and air sealed than
exterior walls where temps are not as extreme.

walls should be insulated and sealed with foam/foil boards to exterior of wall in attic..to stop air movement, temp transfer and to reflect heat back into attic space. sole plates sealed to flooring,
recessed lights air tight. planning on how to heat/cool
at this stage is also a good idea.

sites like southface inst, florida solar energy center and bulding science will show diagrams of thermal/air boundries
these are very helpful in defining where & what types of insulation to use, and where and what type of air sealing should be done.
make sure that you view info for your climate

best of luck.

I would not plan for your mother to move upstairs. My parents put an in-law suite on the second floor of our house and it was a huge problem after my grandmother had a stroke and her arthritis got worse and she was not easily able to get up and down the stairs. A huge problem.

I was thinking one of the teens could move there. Electrical is easy to do later if you are near a breaker box with room on it. Plumbing should be roughed in. HVAC is tougher - really depends where you live. I went a little high on capacity but it was a staged system which made it okay.

Talk to your builder - he has done this before and knows how to do it. HVAC is the most complicated and expensive.

No way would I put a bedroom above the garage, even for teens. It's a lot more of a PIA to heat and cool, and it is never really that comfortable.

I know my mom would much rather live in a community than she would live with me! LOL! If your mother is living on her own now at her age and doing well, then it's doubtful that she would want to give up her independence and move in with you for any reason but poor health. Poor health isn't going to deal with a room over the garage, and may not even deal with a downstairs suite. Poor health is generally an assisted living situation. You'd really have to have room for a home health aide or be willing to do the nursing yourself if you are planning for that situation.

I think that this would have to be a "cross that bridge when we come to it" type of situation. And talk with your mom about what she thinks the next 15 years holds for her. It's nice that you are thinking of her, and helping her, but she may already have the issue planned out.

Sorry I didn't get into details... but discussion has veered off so I'll clarify.

If this were to happen my mother would take my son's room (this is in a one story house so no stairs). My son (now 17) would take the garage bedroom. We would build in a bath with the room. I've since talked to the builder and it is fairly common in this area to do this. We have no codes or restrictions to prohibit it.

We did price building a small guest house but it is way, way, way more expensive than putting a bedroom and bath over the garage.