if at all possible buy a site that already has FPP. save yourself a lot of grief. It will probably be more expensive but less hassle than trying to get FPP for a site. Although depends what county you are in also.
Downside to buying site with FPP is that you would have to go with the house plans as per FPP and not your own design.
Make sure the Waste Water discharge licence is up to date with a site with FPP, have heard of cases where planning granted, but regs changed in relation to discharge licence in recent years and then planning being subsequently withdrawn.