The first step is to survey your home and produce a set of existing drawings to work from. If you already have this information available, you may be able to mitigate the fees for this part of the project, or reduce them, depending the existing information you have available.
You will also need to provide up to date information about your property; regarding ownership and demise of your plot, including any rights of way or easements. If you know, or can ascertain the position of the main services to the property, this will help with its future development.
The following stages are an evolution of this information, increasing in complexity depending on what you need and in how much detail. The more detail you have, generally the more control you have over the project, and the more you mitigate any risks.