I work for a municipality so here is my advice.
I would go to my local town/city hall, Building Department and find out what "Zone" my property was located in. (notice my e-mail address).
After you find out what Zone your in, then you can determine what setbacks, if any, you have to maintain from the property lines. This will tell you how big of an addition you could build.
you will also find out information like, is there a building freeze in your area. If you are on septic, can you add a bedroom, and so on.
While you are there, ask the Building official if he knows of projects like the one you want to do. Ask to see the file & plans.
Officials can't make recomendations, (it is against the law), but, their names are on the permits, write down the names of the builders, electricians, & plumbers who did the projects off the permits.
Drive by the projects and talk with the owners. Ask what they thought about everyone who worked on the job and how close to their estimate did they complete the job.
A good building contractor could draw up what you want. I do not see the need for an architect.
As far as if you have a basement or if you build it on a slab, that is up to you.
Do your homework first then bring in the pro's. If you do, you will be less likely to be taken advantage of.
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