Write down everything you need for the project. Go room-by-room to ensure you're not forgetting anything. Go over the list several times looking for things you missed.
Visit stores, online retailers and speak to friends to get the approximate cost of each of the items on your list. This helps you be realistic in what you can and can't afford. Figure in labor costs if something requires professional installation. Call contractors and get quotes.
Add up all of your cost estimates. If you have a price range on an item or job, use the higher number. Once you have a total, add an additional 10 percent to it. The 10 percent is a padding that protects you against unforeseen costs.
Compare your total cost with your affordable budget. If the total is less than your budget, go ahead with the design project. If it's higher, start deducting items and jobs from the budget that are less important to you. Do not take away the extra 10 percent because that is vital in keeping you within budget once the project starts.
Rework the budget during the project if your additional 10 percent gets eaten up by extra expenses. You'll need to then cut other items to ensure you stay under your projected budget total.