Where to find original building plans for a house

Hi All - want to ask for your advice. Our house is in city of San Mateo and considering expansion, we wanted to find original building plans. It is all original - never expanded since when it was built in 1950. We went to city building department, they do not have those plans. County records also do not have them, but they gave me nice appraisal from 1976 with basic dimensions of lot and house and a teaser appraised value from that era. This appraisal also lists original building permit from March 1950.
Will it be even possible to find original plans? Will those old plans be available from 3rd party companies that have them archived somewhere. Or I will have to hire an architect to create"as-built" plan. Thank you in advance for your suggestions!

You checked the right places. You will have to get as-built drawn.

Original plans from 1950…Fugetaboutit

Thank you for confirming - looks like creating as-built plans is the way to go. Anyone did it? Can you recommend and architect who does it?