I'm planning to convert my detached garage into a guesthouse. Is it OK to reuse the concrete floor? I've seen this in shops and restaurants, and I love the look of those old floors.
A: Old concrete floors are appealing because they're durable, and they often have a nice patina that goes with most any style of decor. I also like how a concrete floor keeps an interior simple, because it's both the foundation and the floor.
My first concern is that patina. It may look great, but what is actually on that old floor? And, how old is it? Concrete is porous, so it can absorb who knows what kind of chemical stains and odors.
Get in contact with a licensed contractor and a company that can test for lead, asbestos, and other pollutants that could affect the air in your future guesthouse. It might be OK, or there may be ways to clean and seal the existing concrete.
It might also be possible that the existing floor and walls are too thin to support required walls for a living space. Again, ask your contractor.
If you need a new concrete foundation, tell your contractor you want to use the finished concrete as your floor. In a living space, the floor should be smoother than one just for a garage. You can stain it a nice color to add warmth, or simply seal the raw concrete with a clear finish to prevent stains.
Concrete floors are naturally cold, so consider area rugs wherever someone will be standing or getting up out of bed or off a sofa. A luxurious option would be to install electric in-floor heating coils in addition to another source of heat.
Let me know how it goes, and send pictures.