Prefabricated bay window kits are available that supply the roof framing, flashing, seat board, and all other needed elements. There is no need to design your bay window from scratch anymore with prefab bay window units available from most home improvement stores.

Answer:  Possibly, but usually not.

Generally, a one-for-one window replacement does not require a building permit. But when you start to increase your house’s “footprint,” building codes begin to apply. True, a bay window does not rest on the ground, but it does decrease the distance from your house to the house next door.

Another consideration is whether you need special tempered glass for the bay window. Because bay windows have seats that permit both sitting and (often with children) standing, this stronger tempered glass may be required to prevent people from accidentally falling through.

Installing a bay window is a pretty big deal, so it’s definitely worth making a call to your local building permitting department to make sure you are keeping within code.