If you want to expand the floor plan, try to keep the addition to a dimension dividable evenly by 2-foot increments—a standard dimension for many building products. In other words, expand your home by 2', 3', 6', etc., but not by 3'-9" or some other odd dimension.
Often adding width to a particular room is better than adding depth, particularly when joists or trusses are running front to back. Adding width simply requires dropping in additional joists or trusses, but adding to the depth may force you into using larger members to handle the longer spans. The advice of a professional can be helpful at this point.
Expanding an existing room is usually more economical than adding an additional room which will potentially require a new door, windows and sometimes hallway space. However, when it is necessary to add a new room, consider the access to it. This can be a tricky aspect in making changes and, in general, a good floor plan will have just enough hallways to allow direct access to all the rooms without forcing you to travel through one room to get to another.
The expensive components in any home are the bathrooms and kitchens with their concentration of appliances, fixtures, cabinet work and hard surfaces. Therefore, adding an additional bathroom, while sometimes necessary, can be an expensive proposition. Even so, one of the most common upgrades people make is in these very areas—additional funds invested here can make a lasting impression and add considerable resale value to your home.
Changes to a two-story plan will often require a little more planning, but, as long as both floors are stacked and you pay attention to how the rooflines will be affected, the result can be just as successful.
A plan may be drawn with the garage doors facing one direction, but you need them to face another. In many cases, when it is a two-car garage (fairly square) on a corner of the plan, it may be a simple matter of just moving the door around to the other side. However, attention must be paid to the structure overhead, so that any beams and headers can be re-engineered if needed.
Before settling on any changes, a visit to the building department is prudent. You will need to verify that any changes contemplated will not exceed any setbacks or zoning restrictions. Also, ask if redlining your plans (plans that have been marked up with changes directly on the plan rather than redrawn) are acceptable. Some jurisdictions do not allow this.