This cost variability depends on factors such as system design, property size, soil type, and system capacity requirements.

Public records are generally defined as those designed to impart constructive notice to third parties. 

So your design plan needs to gently address drainage.Leases rarely a public record.

You haven’t said what amount the rent is but I’m sure it’s not NYC prices and reasonably between 6 adults if they’re working even part time jobs they would likely be able to come up with the rent.

I have a property that I purchased in LA county that has a ~12,500 sq/ft of flat land (12,500 Sq ft lot) with one existing primary residence (1200 Sq/Ft)I hired a architect to design an ADU (3bed/2ba, 1200 Sq/ft) and Addition (3 bed/2 ba, 1200 sq/ft) to the main home. 

But there generally isn't a market for them as such.But that being said once in a while a DST comes on specifically designed for shorter holds and they anticipate an 180 month to 2 year hold. 

I am 59, and my household includes my wife and two adult children.

The city has also placed additional responsibilities on the design professionals but there's basically no barrier of entry to be the building permit holder.

An interior design business? 

I myself is an architectural designer/structural engineer.