Hmm - I find it hard to believe that your garden is full of peat, I suspect the average builder would have stripped any peat out in a heartbeat and sold it on. How do you know its peat? Was it the builders who said that? Much more likely to be something else, I'd have thought - are you on a flood plain? If you are, there may well be waterlogging going on. Otherwise, what have the builders left under the soil in the garden - new builds are notorious for having rubble and debris buried under bought in topsoil to disguise it.

I would be worried if my new house was built on ground like this, it sounds as if you are in a bog rather than peat. Why not go along to your local council and speak to someone in Building Control to find out what is going on?