A lot of apartments are poorly insulated.

You may have to bring them all up to code.Has the building been air sealed or insulated? 

Spray or regular insulation?

While figuring out how to accomplish the renovations we long term wanted we encountered asbestos insulation in the attic that was hidden by the previous home owners( 2 bedrooms were completely renovation and ceilings raised, the master bedroom was still original).

Your codes dept can help with the permits and requirements (insulation, drywall, doors, closets, wiring, window size and heights, including if you need a licensed contractor, professional drawings, the inspections, etc).Keep in mind, if the tenant doesn't have the required number of dependents to get a 4 bdr voucher, adding a bdrm will just cost you money up front and on taxes, but not get you more from the S8 office.

Me and grandfather just built an identical house out of town (32x48) w connected insulated garage for 62k including septic and well which was about 12k on its own.

You need to get a good roofer out and determine if 1) there is any framing rot going on 2) any insulation affected and 3) any sheetrock ceilings affected. 

Unfortunately, their demo already included the garage doors, 6 windows, a half bathroom, staircase, the exterior siding, all the wall and ceiling insulation, and they poured new footings and framed in the garage opening.

If your someplace that has harsh winters, frozen pipes can be a nightmare for mobile homes so I would suggest checking the insulation underneath etc...I think the main difference is that mobile homes tend not to be as durable as a normal sf stick built home so your ongoing maintenance and repairs are usually higher.

They can also chew insulation and wiring and they can be hard to detect.