There are two options that you can follow. The first is to see if you can find out from the current owner who designed the house when it was built, because they should be able to give you a copy of the original plans. If they know who the builder was, he may be able to tell them if they don't know. If the house is an old one then the above is unlikely to be helpful and the only way to get a set of plans is to have someone measure the house and draw them up for you. If you are planning to alter the house, then measuring up and producing an "as built" plan would form part of the brief of your architect or designer.