Appraisals and finished attic space with bathroom

[Jill F.]

Question for flippers, appraisers, or people experienced with appraisals:

 I am about to fix-up my 1910 folk Victorian in preparation for a refi where I'd like to suck out as much of the equity as possible to use for an additional investment property down payment. We have a large attic space that has been drywalled. It has central heat and air and a bathroom already rough plumbed. There is about 450-550 square feet of usable full (or close to full) height floor space. There is a staircase going to the attic but you have to duck on the top stair under an angled beam if you are taller than 5 feet or don't carefully stay all the way over on the right. 

Most houses in the area are 3 bedrooms and 1 or 2 baths with older plumbing ($150-$160k). We have 3 beds /2.5 baths (all new plumbing and HVAC 9 years ago).  I've seen one house on our street go earlier this year for 20% more than the average and one street over where there is less traffic and larger houses (more like ours), I've seen a couple houses go for as much as 40% more.

Would we likely add enough value to make it worth the effort to paint, do vinyl plank flooring, and add a half-bath in the attic?