Go over the actual rooms you might build. For example, a two-car garage might have space for a single-car garage on one side and a small home office on the other. An attic with large windows or a sharply pitched roof could easily become an in-law apartment or artist's studio. Basement space that's significantly large in terms of square footage might become an apartment, too.
The budget is a large consideration. Transforming a damp basement into dry living space will require several steps of moisture mitigation, for example. If you don't wish to invest more than a few thousand dollars, you might not have enough money to properly complete both the moisture problems and construction of new space. Fixing the moisture problems and renovating the basement for storage space only might be a better idea.
Look closely at structural needs. For instance, don't assume you can build rooms in an existing attic without enhancing structural support. The weight of added rooms might cause the house framing to collapse. Size of floor joists in the attic and how walls are supported will factor into the picture. Always ask an expert to help you look closely at issues involving extra weight added to an attic or second floor area.
Invest in fixing plumbing, wiring or foundation problems. A garage floor might need to be completely torn out and replaced if it has huge cracks, for example. Any roof leakage in the garage might signal the need for replacing all wiring in the garage. Never assume you can build a basement bath or kitchen by simply attaching to plumbing pipes overhead. Corroded water pipes or damaged drain lines may need to be totally replaced before any construction begins.
Remodel in ways that add light. For example, install windows on exterior basement walls to bring in natural light. Increase light in an attic by adding two French doors that lead to a deck on one end of the house. Build a family room in an existing garage, but install skylights overhead. Convert the roll-up garage door to a large window space as another option. Simple drywall and hardwood flooring, along with great lighting, is a great basis for any room.