Talk to planning department too about future use. If you are planning to build someday it may be better to have a structure on the property to begin with. Maybe a remodel versus brand new?? Might out weigh the tax advantage. One small example of an issue is a city like Seaside. Tight water restrictions. you can rebuild and get same water credits as you had before...(say 4 bed, 3 bath home with swimming pool...lots of water fixtures) but if you have no proof and the city has no proof it will default to what ever they currently allow which would be a lot less water fixtures and only allow for a smaller home as a result. This will lower the value of the property and once again might not make the tax savings worth while. If you are thinking about doing anything with modular or manufactured houses check in with me.