[Balgair]

I've personally built a few of these granny units in the East Bay over a few summers. It is very DIY-able, but it is faaaaar from simple to do yourself. If you go the contrator route, you had better know the crew personally as supervision is not turn-key.

One example: The concrete pad.

You must know the size of the pad. Then, you have to take soil mesurements for moisture and compaction. If you do not do this, your pad WILL crack as the center's moisture and it's volume changes relative to the soil outside the garder. This is especially a concern in South Bay clay and loam soils. There are many many Levitt-town style houses that are experiencing pad issues. These issues cause the building to shift. Doors will not close, gaps appear in walls, etc. This can happen within 5 years of the pour. Rebar must be tied off at each interesction, otherwise the pad will shift even more so. The garder on the pad must be 24 inches deep. Depending on the size of the building and the load on the pad (which is estimated beforehand) you have to tie the building into the garder with steel hooks (min 0.5' diameter, at least 12' into the pad) due to earthquakes. Concrete's drying period is dependant on a lot of factors, but weather is a big one. Before further construction occurs, the drying must be completed correctly. Actually getting the equipment, tools, and personel to the site is another headache too.

Ok, got that very slimmed down list off the top of my head? Those are just some of the issues with the pad. Multiply that throughout the entire build and there are a lot of little problems to solve and they have very big effects in 5 years time.

So, if you decide to trust your contractor, you had better really trust your contractor. These issues are not simple and there are a LOT of corners that can be skimped on without you knowing.