Since 14 Sept 2015 with FHA 203k
The "new" guideline has some good things inside for those who like Mixed Use buidlings as I do. The guideline used to be that we had to maintain no more than 25% commercial on a single story building, no more than 49% on a 2 story and no more than 33% for 3 or more stories.
We now have had simplified to the structure may be 51% residential regardless of the number of stories, and the commercial use is restricted to the first floor only and cannot adversely affect the residential use. In other words it must be compatible with the residential use.
No smog repair shops, muffler shops, things where there are fumes that might affect the health and/or safety of the tenants above.
This makes live-work more appealing to many as well. Now you can take a a small warehouse and turn it into live-work while taking care to keep 51% of it residential. That might be done with the entry that extends to the residential portion of the structure counting as residential, otherwise the unit may be split into two units.
The mixed use rule has changed to 51% must be residential or more. this is big for single story buildings as it used to be 75% residential.