First off, set yourself a realistic budget and the kind of design you can achieve with it. In terms of the design itself try to stick to right angles, straight lines and simple pitched or flat roofs - however you can still be creative with these forms. Also consider the extent of your current house you may need to demolish or open-up. Working with your Architect to achieve your ideal layout whilst retaining key structural elements will save you money and time at the early stages during the build.