Your budget, lot, or house frame may not allow for you to expand, but you can get some much needed floor and visual space with a well-planned bay window or light-filled bump out. Bump-outs do not require you to alter the existing wall supports or framing plan, nor do they cost a lot of money, but the bit of added space can really open up a room. Be sure to take the time to create an addition that compliments the exterior and historical character of your home so that the inside and outside are improved.
Use rooms for dual purposes to let your home function better in the space you have. Use an upstairs hall alcove as a reading/homework space with a narrow secretary desk with hutch and chair. Use the seldom-used formal dining room as a combination library/sewing room by placing bookshelves that contain doors at the bottom to hold and hide sewing gear. Add a small drop-leaf table and a comfortable chair or two. Use a pantry or guest bedroom closet as an office space, complete with desktop, shelves and roll-out filing cabinet. A pull-down Murphy bed disguised as an armoire makes an office into a guest bedroom.
Remove non-load bearing hallway walls in living spaces wherever possible. This expands rooms by a few extra feet, permits better sight lines throughout the house and allows for more light to disperse through the home.
Use dead space areas like under the stairs, beside chimneys and flues and other little hidden pockets as a place to install built-ins. This will give you more storage and can help you reduce clutter and downsize the amount of furniture taking up floor space. Done cleverly, it can also add charming architectural elements to your home.
An extra foot or two of height creates visual space that can make a small room feel more open and spacious.
Cover and enclose a patio for use as a screen porch or sunroom as an inexpensive way to add additional living space. Hardscape a small courtyard or build a deck and give access to it from a glass door so you can entertain using indoor and outdoor space simultaneously.