Sucking it for the Man isn't cutting it for you? Well, we're glad your standards of life are getting a little higher.
Seriously, besides being a blight on the urban landscape, unused property is a drain in capitalism. That drain is called "opportunity costs". Assets deteriorate when no one maintains them. Having and extra set of eyes and hands always near stops small problems from becoming BIG ones. That's a big point in your favor. You've added value to the property without charging the ohwner a cent. It's also likely an environmental hazard, with stored compounds of various kinds (rubbing alcohol, print toner, electronic equipment) left to rot without supervision -- an attractive nuisance to people like yourselves.
SHOULD you ask first? I mean, before you jimmy a lock (find an open window, etc.) and start building some value for the community to prevent generations of misspent youth...???
And the answer is: not if your an revolutionary (like America's Founding Fathers), but MAYBE if you're not.
Keep in mind, many of these buildings are reducing the real value of the community when they are fully running. But don't guess, do your research. An empty suburban strip mall? 99% likely. Repaint it: Suburban Youth Resistence Center. Remind anyone who comes by of the Boston Tea Party. "Let your sins be strong", Martin Luther said, a good protestant of the status quo (just like your city police's Founding Fathers). If you go "all out", it will be more convincing.
As you can read in Hack the Law, #6, there is no legal or moral claim to probably ANY land underneath any building in America. The only real legal claim that can be made is the building itself and any improved property, like trees planted or irrigation ditches, etc, for these require the expenditure of individual labor. And even then only if the current owner is helping the nation uphold those written ideals we all pledged our allegiance to in grade-school. Guess how many of those are? Roughly zero -- the same amount of moral claim to the land.
But, of course, respect families, people's labor, and of course any Indian artifacts you find there. :^D
So what do you do once you've taken possession? First take care for any liability arguments that could be made against you. Handle any chemicals you find that are a hazard and sequester it or find a local disposal center. Look for any structural dangers either in the floor or ceiling. If you're not sure, find theDoctor or ask a local fire-department friend of yours. You make it more professional than anyone can shake their stick at. You put up a notice that this building is being used for an experimental youth and economic center for community development run by local community members of (some non-profit named by YOU) right at the front window. Print it out so it's professional. If any of the fuzz come by, perhaps with the so-called "ohner", tell them, you are taking possession under IMMINENT DOMAIN (a type of law going waay back to something like the American Conquest) for the sake of the greater community. That you will be providing "fair compensation" because you are implementing a Business Plan. And if nothing else, that you are following the example made by the building owner and the United States vis. that taking possession of something that wasn't yours is perfectly fine. But for the sake of the community, YOU are doing it higher ideals in mind. Get a local PhD who wants to whet his/her chops at defending the ideals of both Philosophy AND America. If you can't find one, then you're just in a dumbass town that doesn't believe in doing the right thing and will probably have to resort to passive-aggressive acts or perhaps just running away.
Keep in mind also, that the market isn't necessarily accurate in determining the value of property. Just because some neo-colonist spent $50k on building a 24ct gold-plated tower of used drills doesn't mean it's actually worth a single penny.
Further, you write out a liability waiver that explicitly states that you will not hold anyone liable for any harm that may come from the property that you are using AND that you will use it for the purposes intended that states why it's for the greater good AND that you will endeavor to keep the property in as good or better condition than can be claimed when you begin (take pictures of the property before you begin). That means YOU are taking responsibility for anyone's safety that may come to your front door. But that shouldn't be too hard. People put themselves in danger just walking across the street or through wilderness, just don't make it secretly more dangerous on account of their own ignorance.
And then, roll out HackerWorldDomination.
This is not legal advice. ;^D