Adding a full-size room on the main floor is generally more costly than bumping out or converting attic areas, but it offers certain distinct advantages. Boosting main-floor square footage with house additions is usually the chief benefit; extra roominess in that part of the house is especially important because it's where your family spends the most time together. A new family room, a spacious eating area, a master suite, or a den/home office is bound to make your home more livable and increase its value, particularly if it lacks these spaces now.
When envisioning a room addition, it's important to factor in other, less obvious advantages. For instance, a new room opens your house in a new direction, with views your existing rooms don't have. Also, the new room will have at least two full-height exterior walls, which present an opportunity to flood the space with daylight. You may even want to add a large archway or cutouts in the old exterior wall to allow some of that light to brighten existing spaces. Sunspaces, one of the most popular types of single-room additions, sometimes take this notion a step further by incorporating skylights or glass panels in the roof design, creating a room that feels almost like an outdoor space.