To build an ADU to help speed overall mortgage paydown - Yes/No?

[Wes Harrington]

So I have been tossing the idea of building a reasonably sized additional dwelling unit (ADU) next to our primary residence for a little while now. The reason for doing so would be multi faceted; largely to house additional family when they come in town, possibly help to speed the pay-down of the overall mortgage (even though we would be taking a HELOC to partially fund the new ADU), and selfishly have been considering a pool with the idea that we could ultimately offset some of the monthly cost from the income generated by the ADU via AirBnB or long term rental.

We have plenty of space on our current land for the building/pool (if it happens). We would be well within what the neighborhood supports both home size of primary with ADU and amenities. No HOA situation. For the sake of numbers, we are looking at a building ~500-700 sqft, built at $120/sqft. ($60-85k). Unit could easily be rented out for $800/mo on LTR, 1.5-1.75x for STR.

We are not dead set on this idea just something we had been tossing around, but our goal is to make a move closer to the goal of quicker home payoff.  I want to know if my thinking has gone as wild as the market!  I am interested in pitfalls of this idea, other associated costs, opportunity costs, etc.

 I welcome all thoughts.

Thanks