Hello KarenSorry you are have this troubleUse and demand heavy type K copper pipe and have it silver soldered if its under groundHave them give you a firm price quote before they start no surprise,no working by the hour or time and material ,,,if you do everyones walking on eggshells worried about the outcome,,,,forget about it,,,get a quoteUse a licensed bonded father and son plumbing contractor recomended from a local plumbing supply house [these folks pay their bills on time and are reputable]Ask them to reroute the piping if they can so most of the new work is not under the slabHave them insulate the pipe with black foam jacket before the backfillForget PVC on the irrigation lines its crap, also install all brass sprinkler valves no plastic,,its Crap..Run your irrigation in Type K copperYour father and son plumbing contractor cam make a recomendation on a masonry /concrete contractor Use them and pay them their priceHave them walk the job with you when its done and explain what they did and discuss future maintenence issues  that might arise they will be happy to do itTake lots of smartphone photos  thru the scope of the projectMake them coffee in the morning and lemonade in the late afternoon they will love you and help you get where you want to goHope this helpsDave DoyleLos Angeles

Specifically, how can we build a unit over a new masonry garage? 

Kendall,Your premium for an Apartment building will vary depending on:1. construction (wood frame, Joisted masonry, CB & Steel, etc.)2.

.; rot, masonry degradation, foundation settling) that won't emerge for several years.

In my experience, things that may have to repaired include (1) water/sewer lines, (2) windows that crack from lifting, (3) sheetrock, and (4) masonry.

Plus I went to school for computer engineering , masonry and auto body.

If you have 8" masonry walls then you need to grout full the wall so basically turning it into solid concrete wall.  

Thank you Dawn dish detergent for getting the job doneWe also have some masonry walls and so we do not allow nails or screws in the walls.

Fortunately we had some Polish warriors to do the masonry work... man they are born masons and become master masons by the age of 12!

And had to more securely attach some surface mounted electric boxes so that the box wouldn't be pulled off the wall when a cord was removed (yeah, they were that loose and walls were masonry / concrete block).