I have a large room that I want to remodel into an apartment for my brother, but first the floor must be raised more than a foot so that it is level with the doorway. The existing floor is a concrete slab, which I would prefer to leave intact. We have major construction experience but don't know the best way to do this ourselves. What do you recommend?
Well, Jeanni, before you can decide how best to raise the existing floor, you should decide which type of finished flooring you want to install in the new apartment. You will also have to determine where plumbing supply and waste lines will be needed for the new bathroom and kitchen, keeping in mind that any new waste lines will likely have to drain (ie. slope downwards) into your home's existing plumbing. Although much of this work may be within your DIY abilities, I highly recommend that you get a licensed plumber involved early on in the design and planning stages so that you are assured a functional and code-compliant plumbing system.
If you want a concrete sub-floor in the new space, then you could add compacted fill and a new slab directly on top of the existing slab. For example, if you wanted to raise the finished floor surface by 12-inches, then you could pour a 4-inch concrete slab on top of 6-inches of compacted fill material and 2-inches of rigid insulation board. In this instance, I would also install a poly vapor barrier on top of the rigid insulation to prevent the poured concrete from getting under (and pushing up) the sheets of rigid insulation board. Keep in mind that you will also need some sort of flashing between the sides of the new concrete slab and the surface of the existing walls to prevent moisture from migrating from the floor slab into the walls.
If you would rather choose a finished flooring that installs best over a plywood sub-floor, then it probably makes the most sense to just frame a new floor assembly (including a vapor barrier, floor joists, and plywood sub-floor) directly over the existing concrete slab. When planning a project like this, you would first establish the finished floor height (which should match the existing floor height on the other side of the doorway) and then measure down to the concrete slab so that you know how deep the new floor assembly must be. If you elect to build with engineered I-joists or untreated wood joists, then you must also install a thick poly vapor barrier between the top of the existing concrete slab and the bottom of the new floor joists. Ideally you will want to install the floor joists parallel to the new plumbing lines because it is easiest to slope the PVC waste line within the joist cavity space and not across and through each floor joist.
One benefit of raising the level of the floor with wood framing, as opposed to using fill material and concrete, is that you won't need to install metal flashing along the lower portions of the walls. In fact, if the existing walls are covered in sheet rock, you may be able to just attach the floor joists flush against the wall. Another benefit of raising the floor elevation with a wood-framed floor assembly is that you can insulate between the new floor joists using blown-in cellulose or fiberglass batt insulation. Make sure to check the Building and Energy Codes in your local area, as they might mandate a minimum R-value for the new floor assembly.
It's worth noting that before you tackle this home improvement project, you should ensure that the existing concrete slab does not have any moisture issues that might cause problems down the road. Unlike a vented crawlspace, this new floor assembly will not be accessible from underneath, nor will there be any air flow to help remove unwanted moisture. Good luck with your project!