No idea without photos and a drawing of the layout--with measurements. You've given us nothing to go by.

Better yet (since you'll have to do this eventually), get a structural engineer out there and have him evaluate the structure. Only he can tell you if it's doable.

Call someone Monday. Done.

You will have to check with zoning regulations because there are specifications for bedroom with respect to access that firemen might need. If there are windows already there, it might be doable.

I second both pieces of advice. Common requirements in the US for bedrooms are a window that opens and is big enough for safe egress in an emergency and a closet. I agree that you need to check zoning for a full list of requirements to see if it’s even possible.

I also agree that you need to get the advice of a structural engineer to see if it can be safely done. For example, the mezzanine may not be able to hold the added weight.

There are other considerations as well. Does it have the proper HVAC set-up for being enclosed? Does the area have power outlets and a light switch/overhead lighting in the appropriate locations? How will closing it in affect the flow and look of your home?

Keep in mind that adding a bedroom will increase the value of your house and may increase your property taxes.