There is no such thing as a one-size-fits-all house addition, they come in different styles and types. Additions can cost as much as you want, even matching the cost of the house itself, when you include a second story, bathrooms, special media rooms, or master suites, etc.
If you want to add just a little more space to your home, consider a room addition. As the name says, they are an addition but only one room (true house additions comprise of many rooms). A solution for many homeowners in both value and cost, they are more than refurbishing existing space but a cost saving alternative to the more expensive full-blown house addition.
When you are contemplating various ways to add space to your house, no other method matches the house addition. You may have an older house, but the addition is space that you can claim as your own. It is like designing a whole new house without the expense of a whole new house. It is 100% your creation, and few remodeling projects are as thrilling and satisfying as working with the blank slate that building an addition affords you.