WASHINGTON—In the Georgetown gated community of Hillandale, residents live in secluded calm governed by some 50 pages of rules banning fences, motorcycles, certain paint colors, tree species and excess dogs and cats (no more than two total per household).

"People come to live in Hillandale because of the quality of our residential community, and that is something that we need to maintain," says resident Sallie Forman.

Then...