Starting the construction of a custom home without a proper plan is a recipe for disaster. Even working on a straightforward production home without a plan is very difficult. It is next to impossible in case of a custom home. Some amateurs builders would try to persuade you to start a custom home project without a plan. You need to avoid such builders and only hire professional builders such as Merlin custom home builders who would not work on any project without a plan.
Some builders would themselves help in the creation of design plan for the home. But, it is always advised to use a professional home designer to create the plan for a custom home. You can ask for recommendations from the builder on a good home designer. Some building firms would have their own in-house designer. If not, you can check with your friends and colleagues. You can always look for home designers online. There are many websites dedicated to home designers nowadays. You can go through those websites looking for the ideal custom home designer to create the plan for your project.