I stay in a new house and the builders are going to put drainage in and
returf the lawn. Problem is that it looks like they are preparing to
put the new turf on top of the old turf. They have already put the sand
down. Is this a problem?

Not a problem. The old turf will just decompose and feed the new turf.
The way most contractors work around here, you probably also just
doubled the amount of loam you have. Be quiet, nod and smile before
they change their mind. Just my $0.02...

--Glenn Lyford