I guess it also isn't a total impossibility that another large quake like the Northridge one could happen again.I have earthquake insurance because it would be a life-altering, catastrophic loss if for some reason I had complete losses or heavy damage (essentially, total losses) on both properties.

(c) The landlord and tenant of a single family residence may agree in writing that the tenant perform the landlord's duties specified in subdivisions (5) and (6) of subsection (a) and also specified repairs, maintenance tasks, alterations, and remodeling.

The renovation/alteration of the house does need to be designed by an engineer or architect in compliance with the Existing Building Code”.

This one is tougher than the typical scenario and don't get your hopes up this will be an inexpensive alteration.

It was an evolutionary process for me and is can still change a little as I change or the markets alter.

What does the lease say about additions/alterations to the property?

You do not alter the property, move things around, tailor it to their specifications - because it's NOT their property. 

Can you paste your repairs and alterations clause here...I need to write into the contract to protect myself from future tenants hiring inspectors or professionals to do stupid tests.How should I beef up my current repairs/alterations clause ....

Basically, I wrote it, then we discussed together and altered it to make it more agreeable to my husband, who wasn't as interested in moving forward, so it was much more conservative.  

There should be something in your lease that says that the tenant is not allowed to make alterations to the property.