People planning an addition tend to picture the build: framing going up, a roof going on, a new room appearing. That part is predictable. The unpredictable part happens months earlier, on paper, and it is where most addition timelines are actually won or lost.
Start with feasibility, because it is cheap and it is where surprises are least expensive. Setbacks, lot coverage limits, height limits and floor-area ratios all constrain what can be built before a designer draws anything. Finding out that the addition you have been imagining is two feet into a required side yard is far better news in week one than in month four.
Next is the drawing set. An addition needs architectural drawings, structural engineering for the new framing and its connection to the existing house, and Title 24 energy compliance documentation. Submitting an incomplete set is the single most common cause of a long permit process, because each round of comments restarts the clock.
Then there is the existing house itself. New square footage draws on the same electrical service, water supply and heating system as the old house, and if any of those is already at its limit, upgrading it becomes part of the project. This is worth checking during feasibility rather than discovering during framing.
Once the permit is issued, the build follows a rhythm you can plan around: foundation and its inspection, framing and its inspection, mechanical rough-in, insulation, drywall, then finishes. Inspections are scheduled, not instant, so a well-run job is sequenced to have productive work available while waiting on one.
None of this is a reason to avoid an addition. It is a reason to start the paperwork before you start dreaming about paint colors, and to work with a contractor who prices the whole path rather than just the framing.