ROI for adding a bedroom...what % is realistic?

[Account Closed]

I am looking to convert a media room (12ftx15ft) into a bedroom in one of my houses. The known costs are $3,500 to put in a window (one doesn't currently exist) and building permit for $150. The room has a closet and a door, so no increased cost to put them in. After checking Rent-O-Meter, it looks like I could increase rents from $3,007 (avg 4 bedroom for my area) to $3,974 (avg 5 bedroom for my area). When I pencil out the cost of $3650 and my expected return is $10,443.60 (after taking the actual rent increase of $11,604 and subtracting 10% for vacancy) it appears that I would have a 286% ROI. The house would still have a room (dimensions 13ftx17ft, with a window, but no closet or door framed out) that tenants could use as an additional living room/playroom/media room right next to the current media room I want to turn into the additional bedroom. The reason for not converting this room is it's large size and the entry to this room has a 45deg angle and seems like it might be better served by not being a bedroom.

Has anyone else had a similar experience when converting an extra room into a bedroom? Is this a practical application to find a highest and best use in a SFR? Are there fees to change the house from a 4 bed to 5 bed with the county tax records?