Some designers, for example, don't offer design change services. Another reason is that it can cost less to use a different designer. Either way, whoever is to make the changes probably isn't anywhere near you, so you will need to specify exactly what you want done. Unless you understand or have experience in construction, you should consider hiring a general contractor. Finding a contractor is easy, but selecting a good contractor can be a challenge.
Builders often tend to concentrate on a specific size price range.
You'll probably have better results with a builder who has experience with homes similar to yours. You'll want to have someone with whom you can really communicate, who has experience with your type of construction, and who is both quality and cost oriented. Click here for a list of tips on finding and interviewing potential contractors. You'll need a construction loan while you're building, and a permanent mortgage when the home is finished. More on that later.