@Ryan Randall  Looks like it might be.New Hampshire Revised Statutes Section 153:10-a (2022) - Automatic Fire Warning Devices and Carbon Monoxide Detection Devices in Dwellings. :: 2022 New Hampshire Revised Statutes :: US Codes and Statutes :: US Law :: Justia

@Nicholas WhelpleyTo assess a property's potential beyond the 1% rule, consider factors like cash flow, market trends, income streams, house hacking benefits, risk management, long-term vision, and professional advice.

Income approaches (direct cap) are most relied upon in MH parks because the buyer is buying them as an investment and its the income stream that you are essentially buying.

If we are planning to occupy the upstairs, what codes apply downstairs to an accessory dwelling?

In terms of your grandfather living there as long as he's not the owner having a dwelling fire/rental property insurance policy is appropriate.As @Greg Scott pointed out he should have a renters insurance policy whether or not he's actually paying rent.In terms of liability coverage, that depends on what you have to protect so there's no one-size fits all approach.

My wife and I have a simple goal of having a long-term mostly passive income stream for retirement.When I do finally manage to get a loan, I probably won't be worried about the mortgage. 

A buddy of mine used Prop Stream and seemed to like it.   

It falls apart completely if you think of yourself as as tenant and pay yourself rent (that you would otherwise be paying someone else).https://www.richdad.com/your-house-is-not-an-assetI prefer the quote from The Richest Man in Babylon, "Make of thy dwelling a profitable investment." 

I think they're selling a revenue stream for themselves. 

While the obvious caveat to this is a slightly higher purchase price (if you choose to do so), there are a variety of benefits to this strategy as well, such as:- The idea of getting a Lump Sum Payday (from the bank) AND a steady stream of Passive Income from you as an investor for the downpayment is incentivizing to most. - If the seller is desperate to close (due to health issues, family problems, liens, etc.), they may agree to Seller Finance the small amount in order to ensure the deal makes it to the closing table. - You're minimizing how much is coming out of pocket when closing arrives.