Add dormer room while replacing Roof

Hi All I am looking at buying a house that needs a new roof. The seller is discounting the price of the house to adjust for the cost of replacementof the roof. My question is, while the roof is being redone, would it be cost effective to add a dormer room to the roof. (Replace trusses, just for that area?). The roof/attic area appears to have enough slopes for me to stand up in the middle (6ft). The dimension of the house is 40 ft deep X 60 wide with currently a tile roof. We are looking to have approximately a 15 x 20 room. The house is cbs construction built in the mid 70's in south florida. Any ideas, thoughts, suggestions? Thanks in advance Ralph
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If you're in an area that uses BOCA or similar building codes, 6' isn't sufficient headroom for an occupiable space. I *THINK* that you have to get to 78" inches for at least half the room, but you should check your local codes, because it might be as much as 84"..
--Goedjn
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That is why I was thinking to replace the trusses where the room would be with a steeper pitch ones

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