1. IGNORING THE SHAPE AND STYLE OF THE EXISTING HOUSE: An addition should be similar in proportions and style to the home it's being added on to (unless there is a conscious choice to make the addition look clearly different). Sometimes this requires breaking the addition into smaller parts so that it doesn't overwhelm the house. Height and rooflines of the existing house need to be considered as well.
2. NOT CONSIDERING HOW DAYLIGHT WILL BE AFFECTED: An addition can make adjacent spaces dark. A room that was once warm and inviting may become dark and cold if an addition blocks existing windows.
3. THINKING THAT BIGGER IS BETTER: There's a temptation to think that as long as you're adding on, you may as well "get your money's worth" by making it big. It's better to design smart, not big. Think about how the new space will be used and make it only as big as is necessary to meet your goals. Consider how existing spaces may be repurposed to work with the addition.
4. UNDERBUDGETING: The cost per square foot of an addition is often higher than the cost per square foot of a new house, particularly if it involves structural changes to the existing house. With any construction project, it's wise to assume that it will cost more than you think. Decide which things are essential and which can be optional if the budget allows. Include an additional 15 - 20% contingency for unexpected expenses.
5. NOT CONSIDERING FLOW: Think about how people will move from the existing house to the addition. How open should the new and old spaces be to one another? Where will furniture be placed? Avoid chopping up rooms with too many doors, which makes furniture placement difficult.