1. The planning & permitting for an addition is extensive.
In fact, it is often as involved as it would be if you were building a new home and sometimes even more so. An addition will require drafting, surveys and plot plans, engineering, development, building, plumbing and electrical permits.
2. An Addition is usually a lot more involved than just bumping out a few walls.
There are foundation and structural consideration not only for the new addition but the existing home. The heating system of your home will need to be altered if not completely overhauled. Power and water will likely be required in the addition. If your basement is finished this may mean opening up drywall and other miscellaneous demo in finished spaces that will need to be repaired later. Roofing may need to be tied in together which may mean re-roofing the entire house. The new siding on the addition likely will not match the existing so quite often the entire home is resided.