You are on the right pathway, my man. You would not believe how many people do not consider the "over-building the neighborhood" thing. Now I won't be able to cover all the bases here, but given time, and considering some of the brain power on this site, you should get most of what you need.
First, some of this depends on your goals. Are you in love with your existing house and it's locations? Does your kids think it is great, or worse yet, your wife? You get the picture. If you want to stay where you are.......that has a "real" value. That value you have to put on the project, especially if in reality you really don't want to move. I mean, when you croak, who cares? If this is the case, then you can free yourself up a little as to whether you are over building the neighborhood.
Another issue, will the existing structure handle the new additions? There is a point when you love the location but not the house and you bring in a D10 and lower the blade.
Now, if we are not in love with this location, and you are not looking at a divorce if you move, then we need to consider some other issues.
Plan out your additions and other modifications required to accomplish your project(s). Then, you are going to have to have some sort of estimate to figure out just what you are talking about in terms of cost......(actually, this step is required regardles-of course). The point is, you have "real" numbers, not vapor to make up your mind. Oh yeah, the usual preconstruction ritual with the city to make sure you can "put" what you want where you want it is required. Check with the inspection department of your community. Anyway, after having armed yourself with real numbers, look around at what the property values are in your area. You can talk to a realtor regardilng this issue........or simply find out what recent sales have brought for homes similar to yours. Keep things "apples and apples". The point is, if your "new and improved" model has a value "less" than the area supports, you are in great shape, if it is more, then you need to consider some of the issues we already talked about. You might need to look into the future, too, to make sure some hog farmer is not going to build in your area, or a turn lane off the interstate that goes in the block next door to service a new hospital to be built in the area.......things like that.
Take a look at it, chew on it, and get back if you need to. People here are glad to help.