"Newer" San Ramon has more conforming homes, most have HOA's, most are two stories, on smaller lots and are further from freeway access. The "older" neighborhoods in San Ramon have mature trees, mostly single story homes (in high demand these days, because less are being built) on large lots. These older neighborhoods also do not have CCR's outlining rules about boats, trailers and street parking. This can impact the look and feel of a street. It can also be a bonus depending on your needs. The homes in east San Ramon run from almost original to totally remodeled, complete with open floor plans like newer construction homes. As far as building on rooms or a small second unit- you need to check with the City of course, but they have encouraged building additional smaller units as a way to meet affordable housing goals. Houses on large lots with a guest house are in high demand as the trend of multi- generational living continues to explode.
Hope this helps,
Prudential California Realty