I am looking for regional information about zones in which guest houses are allowed to be added in the backyards, and also allowed to be rented.
Where can i find it?
Or could you advise on those US states / counties / territories that would allow that?
Like an old Speaker of the House, Tip O'Neill, once said "all politics is local". So are zoning regulations. Every municipality with zoning has similar ordinances, but with local twists certainly in terms of their mapped zones, and often of their regulations (what uses and densities of development they allow in various districts.
So, check City Hall, or their website, or your Building Inspector. Assuming your area has zoning (not all do), if I had to guess, I'd put the chances of what you want to do as less than 50:50, either because of a "one principal building per lot" requirement or, if you're thinking of short term rentals, less than month to month tenancies, ordinances that construe such uses as "hotels" or "guest lodging", a commercial use that may not be allowed in a "residential" area. Sometimes these are covered by "bed and breakfast" inn allowances where an owner-occupied house can have such uses in a residential district.
But this is all speculation. You need to find your local ordinance, see what use district you're in, what uses ("occupancies") are allowed and any supplementary regulations that might apply, such as the typical "one principal building/use" per lot clause in many ordinances, especially in residential districts.
"Use variances" are sometimes available from the ZBA, but these are hard to get; typically you have to show the allowable uses of the property as restricted by the zoning laws would be a "taking" because none of them would profitably really allow you to use your land with a conforming use.
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State, Local, and Municipal Law Attorney
That will very from city to city and county to county. I don't know of any such database, but you could check with planning organizations such as the American Planning Association or similar groups to see if they have any studies or data that might help you. If there is a specific city or county you are interested in, contact the local planning department and ask them what their rules are. They get questions like that on a daily basis and usually are happy to help guide folks on what they can or can't do on their property...
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Land Use / Zoning Attorney
As a past President of the American Planning Association (APA), I can assure you that while APA has certainly produced at least one Planning Advisory Service report that deals with zoning for guest houses that could be rented out, it doesn't have a compilation of which cities and counties allow it. Your best bet is to examine the zoning ordinances of the communities that interest you. Nearly all are available online. Just visit the jurisdiction's website, saunter over to the Planning or Zoning department (in many cases, these will be within a Community Development Department) and look for the link to the zoning ordinance. You'll either get a PDF file or be sent over to one of the online municipal code sites like MuniCode where you can, if you are clever enough, download the whole zoning ordinance. These "guest houses" might be called "accessory apartments." In most jurisdictions where accessory apartments are allowed, there are significant restrictions on them. It is highly unlikely that you could rent one out on a short term basis, like a week or a month. That would constitute a hotel which is likely prohibited in all single-family and probably all multiple-family zones. In jurisdictions that follow sound planning and zoning principles, don't even think about seeking a "use variance." They ought to be illegal everywhere, but in a lot of states the courts have perverted zoning so much that they are allowed. You would be extremely prudent to retain the services of a planning consultant or a zoning attorney to help you evaluate the zoning ordinances or teach you how to read them. It can get complicated.
The information provided in the question posed is not complete enough to provide accurate legal advice. More details are needed to really guide the questioner. My response is not intended to be taken as legal advice and does not establish an attorney-client relationship.
Real Estate Attorney
Some states allow guest houses or accessory dwelling units.The other answers are correct as they very by state to state and local municipal ordinances that control. I would check the local city planning offices or apa website. I would focus an a few geographic areas first and see what they allow.
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Land Use / Zoning Attorney
The location of zoning information will vary from government entity to government entity. However, a good place to start is the government website and the municipal code. Also, it's usually a good idea to contact someone at the city planning department. They can usually point you in the right direction.
In addition you should verify with your CC&Rs if applicable. Some neighborhood associations do not allow property owners to rent.