yes you can do that. First make sure you have the right amount of meters from the house you are in or selling and the one you are going to build,that's important. Then get planning permission. Once you have got this, then part off the area you wish to build on. If you can manage, build the house before selling the existing house (this can help you with connection, pipes sewage..., no complaints from neighbours). If you can not manage you may need to get an agreement in writing with the buyer (on the selling agreement) for permission to build the house and all the possible access you may need from the buyers (best to get in contact with your solicitor for this matter).