Adding a Bathroom
There are two options for adding another bathroom to your home. You can either convert an existing floor space, or build an entirely new room. According to CostHelper.com, do-it-yourselfers may only need to spend as a little as $500 on parts to convert a space into a bathroom, but more high-end bathrooms (with jet tubs and all the amenities) can cost as much as $4,000. According to the above source, the cost of adding an entirely new floor space for a bathroom will start at $25,000.
Adding a Bedroom
According to CostHelper.com, for do-it-yourselfers adding a standard 10-by-15 bedroom, the cost will likely be between $3,750 and $7,500. However, this does not cover electrical work or furniture.
Adding Living Room
The cost of adding a living room or lounge on to your home will vary with the room's size. According to CostHelper.com, a 400-square-foot room (with the dimensions 20-by-20, for example) will typically cost between $10,000 and $20,000.
Adding a Second Floor
Poorly constructed second floors added on to houses are more likely to cause serious harm than poorly constructed ground-level room additions. Add a second floor as a do-it-yourself project only if you have extensive construction and/or carpentry experience. According to CostHelper.com, adding a 1,500-square-foot second floor -- with multiple rooms - on to your home will likely cost between $100,000 and $225,000.
Whenever you add a room or something else of value to your home (such as a swimming pool), your city or town government will usually reassess your property taxes (typically, they will increase). According to CostHelper.com, in most places only the value of the addition will be reassessed, and not your entire home.
In addition to considering the immediate costs of constructing an addition, you must also think to the future. Over the long run, additions -- especially large ones -- will cause your monthly heating/cooling bill to rise. Your electricity bill is also likely to rise, especially if you add on a living room with a home entertainment center and other electronics.