@Ismael Martinez - not sure if it's required to build an ADU in San Fernando, but a bunch of cities in the Bay Area require a soils report from a Geotechnical Engineer for new construction like ADUs.

As for suspended ceilings, it really depends on what is underneath if you sheetrock them. 

NOTICE: THIS DOCUMENT MAY NOT SELL, CONVEY, TRANSFER, INCLUDE OR INSURE THE TITLE TO THE COAL ANDRIGHT OF SUPPORT UNDERNEATH THE SURFACE LAND DESCRIBED OR REFERRED TO HEREIN, AND THE OWNER OROWNERS OF SUCH COAL MAY HAVE THE COMPLETE LEGAL RIGHT TO REMOVE ALL OF SUCH COAL AND, IN THATCONNECTION, DAMAGE MAY RESULT TO THE SURFACE OF THE LAND AND ANY HOUSE, BUILDING OR OTHERSTRUCTURE ON OR IN SUCH LAND.

Case in point: what appeared to be a normal 2-story house from the outside but we got inside and realized that the tiny, cramped central section of the house was a single wide trailer (yep, still had the axle underneath it) and they had expanded (with no rhyme or reason) on every side of the single wide - include building a second story on top of it!

If there’s a crawl space or underneath house search for a product that kills the smell in soil as well.

Interested in purchasing a home and it has “eroding pilings underneath the foundation and It needs to be replaced”.

It's still become very popular but you have to be careful about polluted soils and dig into the background of property in that area even though it's expensive, just a few years ago it was an industrial wasteland and not all of it was cleaned up.

@Jeanne Hink if the numbers make sense to build do that, I'd go with another 4-plex if possible with parking underneath if the rents justify the additional costs and just leave the current units without parking.

Food demand is always increasing and our fertile soils are eroding. 

House build in 76 in clay soil in st louis, Mo.