I think 18 months is a bit too long to make a first complaint about your lawn.
However, it is still worth complaining you never know.
Most house builders use cheap grade "meadow turf" and it sounds like this is what you have.
Most turf tends to die/go brown at path edges and areas like fence posts. This is because the soil is shallower here and dries out quicker.
Sadly this is not a defect and there is little to be doen except water it in hot dry weather.
As you have already tried to lay more topsoil and re seed, I can see the house builder declining any responsibility.
However, there should not be any rubble or builder's debris under the turf or within the top 150mm of topsoil. This is an NHBC requirement.
This would be your best area of complaint.
I would suggest you ask the house builder for an amount of compensation so you can go ahead and get the lawn re laid yourself.
Especailly so if you paid extra for turf to be laid.
Good luck go to the
How to complain section of this website for template letters and guides on how to get your house builder to get defects remedied.