Adding living space by running ducts from the existing system is common practice and does not require a permit.
What you will encounter and should prepare for, is the buyers APPRAISAL. The appraiser may not include this non-recorded space in their evaluation, at the direction of the lender who ordered the appraisal. If you selected the buyer with the wrong lender, you will reduce your price or lose the deal.
Here in Dunedin and Palm Harbor FL, (and most near beach locations) the buyer and seller have a staring contest and the buyer ALWAYS blinks first. Land O' Lakes may not provide the seller the same authority.
Your REALTOR will be able to advise you the lenders to avoid.