If you could bring the sink forward, or take the cupboards backward, it is possible, but your fitter needs to ensure you have head clearance for working at the sink. There may be room to negotiate if cupboards are above the drainer but this might look a little awkward.
Hi forbiebaby, I have got a unit above my sink and it is fine. It doesn't come down as low as the ones next to it but it is usable and we don't hit our heads on it. Been there for a few years now and never been a problem.
If you have a unit over the sink , will you
be able to reach up to get things out ?
Most sinks are under a window ..but your
choice as you have to live with it .
It's perfectly normal to mount a suitably sized wall unit above a sink, Kitchen suppliers catalogues show tons of examples of this. Ask your installer the reason why he says you can't.