Setup :

I have a 90s built, two story, 3bed/2.5bath house that has a pretty large game room upstairs. Game room is currently being used as an office. It's an open concept and you can see the downstairs living area over a railing in the game room. There is also no real hallway upstairs, the stairs lead into this room with 2 bedrooms and bath are accessible along one side. Hope that makes sense.

Current Dilemmas :

1) It's annoying that everything that happens in the house is audible in this room due to the open concept (the TV downstairs mainly). Plus having computers and junk in an openly viewed room looks a little messy.

2) The house is missing a GOOD room to watch TV. We currently watch downstairs, but A) the two story open concept living area makes for lousy acoustics and B) the wife hates having a huge TV and sound system being the dominating focal point of our living room.

3) Lastly, our house is 3 bedrooms, which is one bedroom less than most two story homes in our neighborhood.

We plan to be in this house for at least another decade, hopefully longer.

Our Plan :

I want to wall off the upstairs game room. Wall off the opening to the downstairs and make another wall to create a hallway (bedrooms/bath on one side of this hallway, and this new game room accessible from the hallway via french doors perhaps). We would then use this room as a combination office and media/TV room...sounds weird, but it would fit our lifestyle well. Technically, it would be a 4th bedroom since the room has closet space.

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Any thoughts to this plan? I think it will fit our lifestyle but not sure how it would affect future resale. We would add a bedroom, but lose a common area and the house loses some of it's open feel. We have no formal living room (just formal dining). So our downstairs living/family area would be the only common room in the house unless buyers use the new room the same way we do. So, I am not sure if we are net negative or positive when it comes to resale. I realize we won't recoup our costs, but I don't particularly want to lower the overall value of the home or limit potential buyers.

Can anyone ballpark what something like this would cost to do? Would hire it all out to a bonded/insured company. Along with the two new insulated walls (one of which is curved) and french door, we'd also need to fix the carpet from the damage the walls would cause, run some new electrical/tv cable outlets to the new wall, and perhaps redo the ceiling (currently a tray ceiling that would get cut off oddly with a wall addition). I imagine there are other things I am not thinking about as well in a project like this.

thanks for any advice/insight!