If you're refinancing six properties, bundling them into a portfolio loan might help you consolidate debt, reduce rates, and avoid individual property issues like this.Negotiate Closing Costs or LTV with Kiavi.

I have 2 options:- finance the construction pre-selling the apt, and take my profits (expected to be around 700K pre-tax); or- finance the construction with a 6-7%/year loan (collateral = my house), keep the 10 units, and once the building is completed in 24 months, rent them for circa 180K/ year gross.

The California government will deal with the damage the same way all governments do , they will find a way to raise taxes or get the Feds to pay for it ( that will be difficult with the new administration ) Here is what I find ironic , California has C.A.R.B. to pass regulations on vehicle emissions .

Finally, short-term rentals can offer potential tax advantages, as many expenses associated with the property may be deductible.

They might have insights on specific zip codes or pitfalls to avoid. https://www.biggerpockets.com/forums/521

And, when you see your next tax bill, your heart will stop.

PropStream was good with property, title, and tax data.

Your best advice would come, for a Tax Advisor NOT a Tax Preparer (CPA).

However, it must not have been your or their intention to avoid tax. 

Baselane has the added advantage of synching with your Schedule E you'll use at tax time to file your taxes.