The safe harbor applies to the modification of a building that is not a historic structure if 75 percent or more of the existing external walls are retained in place as internal or external walls, and 75 percent or more of the existing internal structural framework is retained in place.

These are reported and "retained" on IRS Form 8582.Of course, if you are unsure or would like additional guidance in determining what might fit your specific situation, please reach out to a CPA/Tax Professional who may be able to assist.

In some current markets where I buy mainly off-market, I'll continue to use those folks and think I have to pay some retainer fee.Option B) Traditionally how I've gone and most my business will continue to go this way.Option C) Formulating one right now, will circle back when it's more concrete. 

I retain 15% equity in the property and cash flow about $1,000/month

Also, was the Architect retained for construction phase services?

On the flip side, holding onto equity allows you to maintain the proven income stream from your established STR and retain a valuable asset.

I recommend you retain a tax professional.

If buyers ever become responsible for paying my fees, I will probably charge a retainer to ensure I'm compensated for my time even if they never buy a home.

Once approved, we offer a lease to the tenants, collect first months rent (which we retain) and security deposit (payable to owner)  We also make sure they have utilities transferred and renter insurance in place before obtaining keys for move in. 

Common fees will include a set-up fee, a leasing fee for each turnover or a lease renewal fee, marking up maintenance, retaining late fees, and more.