Start by considering what type of home you have, is it a large one of a kind custom home, or is it a home that looks somewhat similar to the homes in your area. If the first is true of your home start with customs drawings. Typically an architect / designer is not required, but one will be helpful with blending into a unique design. If the latter best describes your home here’s a great cost saving tip, start by explaining your room addition to a few local addition contractors. A quality addition contractor will have there own in house architectural design department and have designed hundreds of home additions. More importantly they probably have built many additions like the one you intend to have built onto your home. Ask to see some plans from past projects and once you’ve found a few similar to your project the contractor should be able to tell you what those past room additions cost. This will allow you to get preliminary prices with out the expense of architectural drawings. Just keep in mind that those prices will only give you an approximation of what your addition might cost, but it gives you the ability to decide whether or not to pursue your project any further. If the addition still sounds like a good idea then you need to create a set of specifications. This is the document you’ll be handing to your contractor. It should include every thing the contractor needs to know to price the project. In its simplest form it is a list of all your wants, needs, and material selections. The more complete your able to make this document the better the chance your contractor will be bidding the project correctly.