I have faux dormers that go into the attic and mine has windows. I'm assuming you already have a window in the faux dormer and if so it would be a matter of cutting a hole and frame an opening into the wall that is currently separating them. Add drywall, paint, and extend the flooring into the dormer assuming it is floor to ceiling height. The cost of those things will vary depending on how far away the dormer is from the current wall.

Yes, moderate to extremely expensive

Do you have any idea of the type of roof framing behind the fake dormer? Is there actually head room and space available to make a habitable room? Are there floor joists in place and are they designed for the live and dead loads for habitable space?

If you have inexpensive roof trusses in that area you can forget it unless you are prepared to remove the roof, replace with properly designed floor joists and roof rafters.

If the dormer was really framed in it's fairly easy.

If the dormer was nailed on top of the sheathing it's expensive and major roof surgery.

Thanks for your great inputs. This is a house that we are considering buying, so we don't know all of that information yet, Virgil. They are very good things to think about and ask, though! I am sharing a picture of the roof/dormer, if that helps any. Of course, there is no way of knowing- just based off of the picture -- what is behind it in terms of roof trusses, floor joists, etc. The space in that section of the house is an upstairs bonus room.

That thing is just a little a pimple anyway. It is insufficient and would need to be completely redesigned. Consider that a hallmark of the whole thing. It’s likely designed as storage only, not live loading. Making it habitable space won’t be cheap.

More like a boil that needs lancing!

In any case, as pointed out above, that's a roof not likely to be stick framed. And even if were convertible, zoning may not allow a third floor of any sort.

To convert a fake dormer to a real one, several important steps are required. This isn't rocket science...

1. Completely remove the fake dormer;

2. Cut an opening in the existing roof equal to the size of the new dormer;

3. Add the necessary new reinforcing and roof framing to account for the new opening;

4. After proper waterproofing, construct a new, functional dormer, complete with sealant and necessary metal flashings.

This assumes the interior space is properly designed and constructed before hand to create existing habitable space under the roof.

If this is not the case, go back to Square One, and figure this part out before launching step 1 above.

If all else fails, contact you nearest friendly architect for the necessary steps.

I was wondering the same thing but not for a second floor access but to let light in the first floor.

Of course its posible but whether or not its feasible physically and financially will depend on how the dormer and roof was built.

Rather than dragging up old threads its best to start a new thread with more information than was offered in this one.

When I answer a question with, "Of course its possible", it is usually followed with my question of, "How much money do you have?".