Will pursuing a related degree in urban planning such as public policy as an undergraduate put me at a disadvantage in graduate school to those that studied urban planning?

It depends on what you study though a graduate degree is valued more than an undergraduate degree. For example some jobs look for 4 years experience with a BA/BS or 3 years experience with a MA/MS (made up #s), so do value work experience. That said if you have experience with software or environmental law the level at which you took it may not matter so much. For example, the Intro to GIS course at my school was the exact same class for undergrads and grad students, the grad students just had more work. Initially aptitude with the software or skill is what matters. With all that in mind the earlier you learn something the earlier you can start practicing it. It also occurs to me that if you go to college near where you are from you may have more opportunities open up to you because of existing relationships, not just yours but those of friends and family (hint, use them). You could potentially walk out of an undergraduate program with more planning experience than those graduating with advanced planning degrees and remember, those grad programs are hungry for bright people. Your undergraduate work could lead to a *free* graduate degree or two. In short, explore how the undergraduate programs nearby could bend themselves to your needs. It can't hurt to ask, they may already be in the process of developing an undergraduate program or even a graduate certificate program you could tack on to an undergraduate degree. In the end you aren't the 1st person to ask this question so put it in each schools hands and see what they come back to you with.