A good way to start is by compiling ideas and inspiring images to be passed on to the designer. Keep in mind your budget and prioritise the elements of the design that are most important to you. You can measure furniture and display pieces that you know you will be requiring in the new build. Think about outdoor areas and gardens and how you might like them incorporated into the design. Kitchens and bathroom layouts can be a very personal decision - consider if it is to appeal to others. Be as comprehensive as you like - there are many elements that make up a home and discarding an idea may be easier than making a rushed decision at a later stage.
