You can pull the permits on line. Go to :https://www.velocityhall.com/accela/velohall/search/index.cf
You can check what permits have been pulled and the value of the individual project.
Keller Williams Realty Palo Alto
Therefore, in reality, your best bet is to check with the homeowner or architect of the project to see how much a particular project cost them. Do not ask the builder, since the builders tend to progressively increase prices during successive projects.
Builders love to be contacted about possible new business. If they are doing work nearby it will be easier to get them to stop by and just chat with you. The quickest way to get a good estimate would be to call them, tell them you are next to one of their job sites and would like to know if $xxxx is a good ball park number.
Some very rough numbers to work with would be $250+ per sf to add on a bedroom. The cost of labor to replace or install a new window is roughly equal to the cost of the window. The cost of a bathroom or kitchen remodel varies dramatically (triple, quadruple ...) with exactly what you want done.
I have a short diary posted on-line showing my experience building a new house in Palo Alto.
Palo Alto has most newer building permits posted online. You can research them from:
Many projects encounter "change orders" so the cost listed for the building permit may not be very accurate even after adding all the sub projects together. (You've surely heard about the cost overruns for our library).
Another aspect to consider is how much value will be added to your home after doing the work. It can very dramatically. Adding one bath to a 3 bed 1 bath home can add more value than the cost. In general you want to be very careful if you are doing remodeling to increase your home's sale price. For Palo Alto I have fairly good statistics showing the increased sales price for homes which were torn down then rebuilt:
Top 3 agent nationwide at Keller Williams Realty
Over 25 years experience
Over 1,000 home sales in Santa Clara and San Mateo Counties
Try looking at the City of Palo Alto's Development Center web page:
HB Roswell Realty