I'd call a local contractor.

A rectangle garage is “simple” enough, that if you’re not looking for detailing your match the house, then you don’t need an architect involved. But, if you do want a building that looks great, and echoes the house detailing, an architect can help you with that.

Does he have a transit and can measure the future site? Because “flat” isn’t usually flat. That’s the biggest issue that people don’t realize, until they try to fit garage plans to the actual site . Garage entries need to be at grade. If the ground slopes away, you’ll need compacted fill at the rear. If it slopes down towards you, you’ll have to remove dirt and degrade to keep water away.

Siting the garage needs that information, as well as required setbacks, utility paths and easements, etc. Even a simple rectangular garage can end up pretty complicated to build unless your on a large dead flat lot, with no utilities interfering with anything. Which brings us to how you plan to get electrical service, or any other service, to the garage. You have to figure that out too.

Find a good local contractor, and talk to them about what you hope to achieve. The more time and money you spend with a pencil in hand rather than a hammer, the better the job will be.