If you're talking about the 'legal considerations' of when it becomes spam, if they "opted-in", you can send them whatever you feel like as long as you have an unsubscribe at the bottom along with your physical address.
CANN-Spam regulations can be found here with more info --> http://business.ftc.gov/documents/bus61-can-spam-act-complia...
That said, you don't want everyone unsubscribing so you need to focus on making sure your emails are not "perceived" as spam. Perception > Reality.
Basic [very basic] tips.
--Make the emails useful (give people inside tips on places to park, ways to avoid fines, hidden parking gems of the week)
--Don't make the emails all about you. They signed up for your list because they think you can help them with some sort of problem. Help them solve part of their problem in a small way, prove you're an expert and then when you launch you can let them know that you can solve a much bigger portion of their problem
--Segment your list. Find out who's opening up & clicking through your emails and who isn't. Send the people who are opening & clicking more email. Send those who aren't less. There's a lot of talk about sending too often or not sending enough, but that's a bunch of crap. Fuel the fully interested customers with more content. Don't harass those customers who are partially interested with daily emails. Segment, segment, segment.
--Make your news Newsworthy. When you do have something to say, make sure it's worth saying. Just sending something because you think it's newsworthy, doesn't mean it is. Make sure it's something you would actually want to know about the company. Your list's attention is precious. Don't abuse it.
--Have a personality. Get someone to read the next word. That's the name of the game with email/content/copy writing in general. get someone to read the next word, then the next one, then the next one. Personality helps with that. There are people who sign up to groupon and don't even care about the offers, but they'll read every email because they're entertaining.
Hope that helps. Best of luck!