Help subdividing 10,000 sqft lot of 4.8 acre property to sell.

[Hieu C. Nguyen]

My primary residence is on 4.88 ac and located within OKC limits. I have a stone barn on my property. I want to split off and sell ~10,000 sqft of land that includes the barn to my brother so he can renovate it and live in it. The barn currently has electric and central heat/air. It does not have plumbing and I've been quoted about $12k to connect to city sewage and add plumbing. The barn already has a separate driveway with street access. 

I have read that there would be issues with the bank if I split the land since the deed on the mortgage includes the whole land. I assume I would have to get it reappraised and get a mortgage that doesn't include that section. How would I go about doing that? 

Do I need to refinance first and then split or split and then refinance? I tried calling the planning commission but haven't heard back yet. Do any of you guys have experience doing something like this? Or even just subdividing a lot with a mortgage?  

Thanks in advance.