Prefab or build ground up granny pods

[Andrew Jones]

This goes against my beliefs, but we're going to spend money on real estate that is not for profit.  My mother is moving in with us and we're looking to buy or build a small 700-1000 sq ft home for her.  She travels a lot so this would just be a quite place to pack and unpack her suitcase between adventures.  We have a half acre on a corner lot and we're planning on demolishing the old horse stable (the parcel used to be much larger) and running plumbing, electrical, etc.  I have two questions for anyone who may have done something like this before:

First is should we build from the ground up on site or buy a modular?  We have young kids so we'd prefer not to have long term construction going on in our back yard, but this is for my mother so we need to do it right.  Modulars we've seen are actually much nicer than I would have guessed, but what should we be looking for in a builder and how much should this cost?  I want a nice fit and finish for the unit (granite, 2 bed, 1 or 2 baths with laundry).  We're in the Sacramento CA area if anyone is familiar with cost to build out here.  We'd also be interested in referrals for prefab homes and/or builders.

Second is what we should expect to spend getting the pad ready.  I can rent some equipment and demolish the old building myself, but what should we expect to spend getting a concrete pad poured, wiring and plumbing run the 100 ft or so from the main house, etc?  I've never built/purchased a new structure before so any advice would be appreciated.