With any kind of window providing a seat, especially bay windows, the most obvious storage add-on is the window seat over a storage bin. With very wide bay windows, building in a window seat bracketed by shelves can create comfortable, workable table or desk space as well as extra storage area. A hinged seat can cover an area perfect for storing blankets, slippers or other useful items that often clutter rooms.
Smaller under-window areas require smaller storage ideas, the simplest of these being shelves. If you have the right measurements under your window, inexpensive pressboard shelves from a department store are the easiest and quickest solution. If the space beneath your window is an odd size, you may want to build your own small shelf unit.
Sometimes you have an exceptionally deep space beneath a window that is inconvenient to access. Using lazy Susans and pull-out units can bring all that space to you, while keeping your clutter organized and hidden away.
Lazy Susans are particularly flexible space solutions. Sold at Ikea, Home Depot and dozens of online retailers, a D-shaped multilevel lazy Susan hidden behind cabinet doors can make the most of an odd-shaped space beneath a bay window. In areas where a lazy Susan doesn't make sense, pull-out organizers instead of drawers can give you space for storing smaller items like gardening implements or cleaning supplies.
If you have a large appliance such as a space heater or vacuum that needs a convenient hiding place, installing a simple one- or two-door utility cabinet might be the best use for that under-window area. This is especially useful for hall areas or a foyer.
In bedrooms and dining rooms, drawer space is a great use for the area under your window. Wide, shallow drawers beneath tall dining-room windows are great for storing tablecloths and serving platters. In gabled bedroom window areas, a dresser or built-in drawers can greatly expand the storage area available to children or guests.