We did one where just about every door frame was 1/4inch out of a std size door width & the heights varied from 78.5" to 83". 

Everyone is enjoying prices at the pumps at 20 year lows (adjusted for inflation), everyone except for small and medium oil & gas producers that can't afford to continue operating with their existing debt and the price of oil at $30 a barrel.

Is there any law for minimum width for stairway? 

My mission as stated above is to build a client base of steady customers who call on me again and again for all their small to medium sized repairs and improvements. 

I am willing to do small to medium renovation, but nothing to drastic at the moment.

What may not come into play immediately are updated hallway widths or number of outlets and overhead lights, those things may only be asked for when you remodel.  

This is due to the fact that it will likely cost you around $80-100/ft2 (this number is very regionally specific, but should be close in your area of FL for smaller to medium houses, 1200-1800ft2) to build a new house now.

Things like the density are important (4 units/ac), but what about minimum lot widths, frontage, setbacks, mitigation or environmental?

I am quite handy and can do a lot of small to medium fixes myself, for example I have finished a 650 sq/ft addition on our primary home and I have really enjoyed being in a General Contractor role.

After you look up what medium of owner financing you agreed to that is.