I've had a couple of greenhouses about the size you describe. Even with a greenhouse heater I could not afford to heat one in Salt Lake City to the point where I could keep houseplants like African violets and orchids all year long.

Cooling was often a problem. Too hot in the heat of the day even when heat was needed at night.

I would not pass up the opportunity to have another even if I only used it as a potting shed and a winter sowing facility. Greenhouse are fun and handy in spite of some problems I have mentioned.

Permits of course are very, very local things. I've neighbors with sheds that are no longer allowed, "grandaddyed" in as they say.

Closes to a liability problem I ever had was when a Russian kid moved into the neighborhood and the Mexican kids persuaded him to climb onto my greenhouse to steal apricots and he fell through the roof. Fortunately, legally speaking, he was unhurt and he was the first ESL kid in the neighborhood to learn the word apricot - in English.

You could always use the greenhouse as large coldframe. I'm sure it should heat up enough during the day to stay warm during the first few frosts...it'd be great for those last few tomatoes or those tropicals that are still full of buds (like brugs etc.) You could sow lettuce or other cool crops a few weeks before they can be sown into the ground.

Even without keeping it heated, a greenhouse can be great as a focal garden structure (a rather large one at that!) decoration or you can just use it as a potting shed.

As for permits and stuff, like Albert said, it is local and the rules change from town to town (not to mention state to state). I do know of some people up here who have unheated greenhouses, and they turn out to be just as useful as heated ones.

I use a large "pop up" unheated igloo greenhouse ((8x8)to start my annuals and dahlia tubers and caladiums and coleus - - anything that wants warmth before mother nature is providing it in my area. I only keep it up for Easter to Memorial Day, but it is enough time to give warmth loving plants a start.

I used to have a greenhouse....we moved and it didn't...I kept it heated with a space heater. I had it under the potting bench, and wraped the bench with a tarp. I stapeled it to the bench. But once I started wsing, I stopped using it for starting plants. But it did make a great place to get my wsing done. I was out of the cold, and just warm enough inside to make it bareable, and no mess inside the house. I had so much more luck with wsing, I could never get anything past the dampening off stage. But if it's free, even if you have to move it, and put it back up, and don't mind all the work, I say go for it.

I do like Nan and use my hoophouse in the last month before the final frost, just to get annuals and tomatoes going ahead of schedule. I ate my first ripe tomato in early June this year. I'd never try to heat something here in my zone either. I just think it's a waste. I keep tender things inside under lights running 24/7. They don't grow much, but they do survive. In spring, I move them into the hoophouse and shade it with a white bed sheet to harden them off. My pineapple sage was done this way.

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I also sowed annual seeds in styrofoam cups. I put concrete blocks in the base to hold heat for the cool nights. It worked better than containers on the ground for zinnias and red salvia.