Most unpermitted living area improvements are made by either enclosing a patio or converting a garage. With garage conversions, “curing” the unpermitted area tends to be pretty simple – they go in and remove any partitions, take up the carpeting and make the garage door functional again. Patio improvements tend to get ignored (in terms of value) by appraisers and realty agents.
Full-on frame/siding additions with foundation are not nearly as common. Ironically, it’s often contractors who put those in their own homes – they know how to go through the permitting process but they just skip it. These additions are generally built to code and can be permitted after inspection and testing by the city’s inspectors, if that city is one that will allow permitting after the fact.
It’s the handy do-it-yourselfer you have to watch out for. These guys watch Bob Vila on TV and think they can figure it all out from one of the home improvement books that Home Depot sells. They have good intentions but they do tend to make mistakes. They tend to look like crap, too.