So some homeowners have to upgrade the foundation and make sure that the units are watertight.

Missing flashing can be tricky to replace and make water tight if you are not experienced.

The water should be water tight, the p trap stops the odor of sewer gasses, and it all goes down the drain to the same place.  

Kind of like the watertight compartments on the Titanic, or another ship (bad example I know)  The person giving you that advice just wants their holdings as compartmentalized as possible and maybe isn't considering the cost of maintaining all the LLCs.

@Dean Gittings i have sheds on a couple of properties and the lease says “sheds are not warrantied to be water tight or pest free.” or something like that.

You can find vinyl sticker tiles as cheap as $0.25 here where i live, but you get what you pay for.It is easy to replace if/when a tenant messes it up, which is nice.I will say, I'm not a huge fan of using these tiles in a bathroom, because in my experience, they are difficult to make water-tight.  

Any way I will be back out end of oct.We have 9 new construction starts in Aug so about end of oct we can see water tight framed in homes..

I have never seen it done, but it will be watertight and you can keep your windows.

It is an OSB type material that is saturated in resin so it won't absorb water while buildings are going up but not water tight yet.

One question, is finding a competent lawyer / attorney the first thing you should do to make sure your contract is water tight?