If your home can sell below the current market then that difference represents the amount of remodeling you can safely perform and still expect to get a return on your investment. Over that amount may require some hard selling on the part of you and a future real estate agent. Never start a project without an approved design and materials list. Always make sure that your local planning and building department approve your design before you sign a contract to build. The same advice holds true if you are planning on spending more than a few thousand dollars.
Good architects are worth the money and they can make all the difference.
Consider your financing. Make sure your budget and your plan are compatible. Also, make sure your homeowners insurance policy reflects the increased value of your home when your job is completed. Plan your time. The more time you give yourself for planning the better the over-all result.