The above are all good, focusing on legal, zoning all a huge variable per the locale/city/county.The practical is re NEW separate power meter, NEW separate water meter, hopefully NO gas to duplicate. 

Lots of variables to consider when coming to this market.

Signed a purchase (600k, 1650sqft single family home in Folsom ranch)contract last year, plan is to get the down payment from heloc(2.5% apr until next year jan, after that it's variable).

There very well could be a sector that sees the possibility of a 1% rule, but there are just so many variables in a commercial transaction and deal to take into account I doubt there is a quick check as clean as a 1% rule. 

The promo lasts from Aug '22- Aug '23 & we pay the variable rate for 1 year & then sell the California house in fall '24 to pay off the HELOC.

Keep shopping around or get with a mortgage broker to shop for you.NWCs add a variable but are still able to be reasonably financed.  

But as Brandon mentioned, there are a lot of variables that go into this.Brandon also hits on some of the faults of logic in your idea.  

I'd rather keep my 2.5-4% interest rates on a majority of the debt and access the equity with a HELOC at a higher rate, essentially creating a blended interest rate of 20% variable, 80% fixed.

I recommend you do this each year to adjust for utility increases and other variables.

Are the ducts zoned with variable dampers?