Planning to build a 3-season cabin in northern WI (sandy soil, 100' from a
lake, weather extremes but no earthquakes). To spread the costs I'll get
by with a sand point well and a privy at first, add indoor plumbing a few
years later.

One option is to contract with a company that builds garages and cabins.
They frame the walls in their factory, erect on a reinforced concrete slab
over 12" sand buildup. They've been doing these for 20 years

Here come the questions. When it comes time for water and septic, I can
think of 3 options:

1) Break up the floor and somehow run plumbing underneath. Is this the
usual practice? Is this even practical?

The other options would mean putting up a small addition (I may be able do
this myself) to hold the pressure tank, water heater and bathroom, route
supply and drain lines through the wall for the kitchen sink.

2) Build this on another slab. Do I need to tie the 2 slabs together and
how is it done?

3) Build it over a crawl space (to hold the tank and heater). Is it even
possible to tie the 2 structures together -- one on a slab, one with frost
footers -- and how?

And what else am I missing here?

TIA