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.
One-room additions pose certain design challenges that need to be addressed before you firm up plans. First, it's easy to miscalculate the amount of floor space you'll actually need and end up with a room that's either inconveniently cramped or uncomfortably barnlike. For best results when planning house additions, sketch a furniture arrangement to scale on graph paper, then add traffic lanes where you think you'll need them. Be sure to allow for door swings (including those on cabinets or appliances), space to push back chairs from desks or tables, and special spots for holiday-related displays or activities. Draw walls around what you've sketched, and then calculate the square footage they enclose. Next, make sure the new room addition will be properly heated and cooled. Maintaining comfortable temperatures is more difficult in a room with three exterior walls than in a room with one or two, especially if you live in a four-season climate and want lots of windows in the new space. If you're adding a modest-size room (10x12 feet), your current heating, ventilating, and air-conditioning (HVAC) system can probably handle the extra load. For larger spaces, consider installing extra ducts, a heat-circulating fireplace, or an additional small HVAC unit.