Re: 2nd story or ground extension
If you do build out rather than up, you can also incur development or impact fees due to the increase in non-permeable surfaces (that is, roof rather than lawn) that require additional investment by the city to deal with additional runoff. In some locales, this can be significant. You may also be subject to setbacks, where any new construction must be a minimum distance from property lines or other features such as wells, septic, or utilities.
Building up, you may face restrictions on height. You might impact your neighbors' view, which can affect their property values. Then they'll sue you or convince the city to issue a stop work order.
If you are in any kind of historic district or HOA, expect a whole host of restrictions on what you can do.
Involving your neighbors in the planning process will help things go smoothly, since just about anything you do on your property affects them in some way.
The "Senior Member" designation under my name doesn't mean I know a lot, it just means I talk a lot.I've been a DIYer since I was 12 (thanks, Dad!). I have read several books on various home improvement topics. I do not have any current code books I can refer to. I was an apprentice plumber for two years.