Other option would be to sell one of these homes and buy a single family home on a larger, flatter lot with more potential to build, but then I lose these great mortgages I've got.  

Feel free to inquire about their staff qualifications if it's a larger organization.2.

Also make sure to build on a property located in a desirable location with larger pop density and if holding for rentals, charge median rent for that market as lenders want to see a bigger pool of buyers/renters available to occupy/sell. 

@Matthew Posteraro I wouldn't necessarily say that it is a way to make them more hands-off by scaling up to larger properties.

Handle Legalities:Make sure you have all the permits required for the task you intend to perform.

What I recall from Aaron's deals was it wasn't about finding ever larger seller financed deals, but keeping the old seller as essentially a PML....keeping you from constantly looking for new funding when you already have someone that agreed to fund you essentially. 

Others put down larger downpayments to make the cashflow work.

Leverage magnifies return. 3) The effort involved in adding an ADU is comparable or larger than a rehab associated with a BRRRR.

Leave the owners alone, when and if they are ready to sell, they will, and there will be plenty of lots to choose from on the MLS.I have no statistics to back this up, but in my casual observation after our fire is about 1/3 of the owners rebuilt their homes (some built similar, some duplicated exactly--big mistake, and some built better/nicer/larger homes). 

I do know managing more than 1 property is a tall task unless that is pretty much your job.