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That is going to vary depending on where you live, the particulars of your home construction, as well as particulars of your planned "pop-top". Here in Richmond, VA, the structure and exteriors, plus a window or two for a relatively narrow dormer runs in the $10-12,000 range... before you make interior renovations to the bathroom which presumably you may be seeking.
I am guessing that you may be interested in something wide than a 3-4 foot typical dormer. Well, things become much more structurally complicated in a Cape roof, the wider this pop-top becomes. As the roof loads become asymmetrical, beams and opposing side structural work will likely become involved (ie: you may have to add structure on the OTHER side of the house, and a beam at the peak of the roof... possibly posting-down to the foundation, to balance the load). I have seen those go to $30,000+... again, before you start adding in the bath renovations below. I know; sounds shocking but in order to maintain the structural integrity of your roof, these things may be neccessary.
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