A dormer is an extension, a building addition connected to the end or "tip" of an upper room; it's constructed on to an already completed house, thus adding more space to the room and usually another window as well. It's surprisingly easy to build (or install, if you get a prefab dormer), and also surprisingly inexpensive, once you actually work out a dormer's cost.
A standard prefabricated size for a dormer extension is around 5' x 7' in width, 8' x 10' in height. At that size, it should be easily fitted to most upper bedrooms, to add either more light or more storage space (it's ideal for a child's cubbyhole, for example, unless you have a lot of children).
You're actually adding another room to your attic or upper floor, but here's the exciting part: the price. If you're incredibly handy with those power tools and can get the wood on sale at Home Depot, the average do-it-yourself price for a dormer addition is between $1700 and $2000 in materials (this assumes you have a circular saw and framing squares among your tools already).
Even better, if you're in the process of building a home and decide to add a dormer, and a contracted crew is already engaged for the work, on average the crew adding the dormer to your home during the construction process will add only $2000 to $3000 to your total cost. This assumes you planned far enough ahead to add it to the design as you and the contractor are incorporating all the other elements before building.
Now suppose your house is already completed, and you want to tear out a section of room and put a prefabricated dormer window piece in place (which will probably require a crew, since that’s quite a job even for a do - it - yourselfer unless you’re also a house - builder). Assume a standard size, double - wide window piece is being added on; the lowest estimated figure is about $2500, and the higher - end (and far more likely) figure would be $5000 and more.
At that price, it's still a lot cheaper than building a ground level addition or garage.
An open dormer, such as a window piece without glass that might fit on the roof of a barn, is even cheaper; Farmer Brown won’t have to shell out more than $1000 in materials, or double that if a contractor gets hired. You can store a lot of hay in a loft like that.
Want a gable, like Nathaniel Hawthorne's haunted house in House of the Seven Gables? That's where it runs into a little more money, since there is a great deal of variance in price depending on the wood, design and "swoop" of the piece. It's definitely a contract job, and costs anywhere between $15K and $25K. No wonder Hawthorne had nightmares with seven of them.
That should give you some idea of the price range and affordability of this charming addition to your home’s upper levels. Unless you're buying prefabricated and very high - end products, your new dormer can add charm, intimacy, space and light to an upstairs room for very little money.