We will know in about a week how accurate my 85% estimate is. 

You will need to talk with a local builder to help determine feasibility and rough ball park estimate of construction costs.

Don't just estimate $100.

Does anyone know a good resource for a reliable estimate of water/sewer/heat in this area?

Clearly, you're looking to househack, which is a great way to start.Does your rental estimate include the owner unit or not?

I have been estimating $125/sqft.

BTW, I'd get another estimate to clear a blockage--$2,500?

Each investor is going to have to think about their own financial situation, the location, asset class, type of asset, specific tenant, and their own best estimations of the duration of the pandemic to arrive at their own policy for how to handle l late or missed payment.

I would estimate our house decreased in value about 10k recently. 

@Jonathan R McLaughlin: I appreciate your response and your estimate of my limited knowledge of history, concerning your examples, is a fair one; thank you for those examples.