The key decisions to make at this stage include working out what you like and what you will need exactly from the house. It sounds simple but this will actually take up a lot of time.
Then you need to find the right design partner to guide you in making the house both Building Regulations compliant and efficient. After that or in parallel, you will need to start thinking about what building methods you want to employ, how energy efficient and environmentally friendly the house is to be, and of course you’ll need to define how much money you want, and can put into the build.