you will need to submit a new application, I think it would be unlikely that it would cause any major issues once there are no restrictions on the site. I changed house plans on a site previously, no problems.
Just make sure house design is acceptable to planners.
If you want to be certain sure before you purchase site, arrange a meeting with the planning office or get a local councillor to enquire for you.
you could also make it condition of purchase that it is subject to change of house plans.