Prefab may be worth looking into, stick built is worth looking into.

Unless you own the needed crane, and the construction company, prefab won’t save you a dime. Especially with a site access difficulty type issue like an ADU in a tight urban area. Time? Even that is doubtful.

Hmm..boxabl folding house? Purportedly $50k.

Pre fab will likely be less in the greater bay area, but don't forget to get costs together for permitting and connecting. Sewer connections can be a large impact if the only opportunity is around the house to the front tap under the driveway, easily surpassing the number shown below for the "box".

SJ has pre designed resources here:

https://www.sanjoseca.gov/business/development-services-permit-center/accessory-dwelling-units-adus/adu-permit-plan-review-process/adu-single-family-master-plan-program

Prefab usually saves time more than money. The factory part can move faster, but the site work is the same either way: design approval, foundation, utility trenching and the sewer connection.

That last one matters in San Jose. If the sewer tap is at the street on the far side of the house, the trench can cost a lot no matter what type of unit you choose.

Access is the other big factor. A modular unit needs a crane and a clear path to the backyard. If your lot is tight, ask the prefab company for a site visit before you get excited about a price.

To compare fairly, get a total installed price from each option, including foundation, utilities, permits, delivery and crane, and finish work. Compare those totals, not the box price.

In California, an ADU of 750 square feet or less is exempt from impact fees, so check whether sizing to 750 or under fits what your mom needs.