I think sitting rooms in master bedrooms are useless, so for me, converting it to another bedroom sounds like a good idea. Particularly since you're actually going to use it as such, not just occastionally, but all the time. Is it large enough that you will be able to put a closet in it? Without seeing a floorplan I don't know what the degree of difficulty is, but in general, adding internal walls and no plumbing is one of the least expensive type of change you can make in a house. What type of flooring is in the sitting room?

Expense is relative ... this, as far as remodeling goes, is not an expensive project. Can you include a diagram of the space? Are there windows? Also, you'll need to make sure to have enough space for a closet.

Here is a photo of the room taken from the master bedroom. The new opening would need to go approx where the artwork is hanging on the left. Because there are built ins I don’t know if there would be space or need for a closet. I recognize that it would not be considered a bedroom without a closet but it would work just fine for our needs. The room is 11x10 and it’s carpeted but we are looking to replace all our upstairs carpet with hardwood. I think the casement windows would be big enough for egress but I would have to check local codes for that.

I’d absolutely do this. In the future, it can be an office, or gym room, or yoga room or whatever if you or a future owner don’t need it as a bedroom. Is it 11 x 10 with or without the built in? Are you trying to keep the built in?

I think it really depends on the kid getting the space if it works . A girl needs a closet and alot of closet . I see not use for the builtins so why not make that space the closet you might even be able to keep the bottom cabinets and add hanging space above. I do agree these areas in bedrooms never get used . I have converted many for clients into really nce dressing rooms . I guess alot depends e ven on the age of the kids.

You will want that private sitting area when there are teenagers and young adults that take over the house.

I would do it.

I would definitely do this. Think of the years of peace between kids with their own rooms.

I think this makes sense since you intend to live there and it suits your life style

If people don't need five bedrooms, then it can always be used in some other way. For example, many people would prefer to have a home office that is separate from the bedroom or even just have a "sitting room" for the parents that isn't part of the primary bedroom so that one person can use it without disturbing the other if necessary.

The only question I have is how you would build a walkway as that might entail construction issues which makes it more complicated than just putting in a wall and a door where there is an existing hall way to enter from.