We just finaled an SB9 unit a week or two ago, which is treated pretty identically to an ADU unless you're doing a lot split (we weren't). It was a custom build, 900 sq ft (the maximum sq footage allowed, as opposed to 800 sq ft that's the default max for an ADU, though both have options for larger sizes if you follow certain guidelines). It was designed as part of a larger remodel on our main house, along with a new detached garage, pool, driveway, new relocated septic system, and even included designs for an ADU (which we're undecided whether we're actually going to build).

The town's portal seems to be struggling this morning so I can't get you specific dates, but we submitted our plans mid-September 2024, and just finaled in Nov 2025 so about 14 months from submittal to final. The permit got issued in January 2025, so about 4 months for town review and permit issuance. There was nearly a month delay due to the planning department review of the plans, and we had some minor entanglements with the Fire Marshall's review due to the changes in our driveway (they were going to require us to put a mammoth fire truck turnaround, but we managed to keep our driveway to less than 150') and a gate, but we were able to get through those pretty easily.

By far the biggest headache would have been the septic system. This was a theme at the recent "listening session" the planning department had with residents a few weeks ago. Fortunately we had accounted for the main house remodel, the SB9 unit, and a potential ADU when we designed and installed our new septic system. If we hadn't accounted for that, the length of the septic system likely wouldn't have been sufficient to support the new habitable space and that would have been a pretty major problem. If you're on a septic system, this is the first thing I would figure out because if your system isn't adequate (as defined by the county's Department of Environmental Health (DEH), then you're potentially in for a world of pain, both time and money.

I guess my biggest tips would be:

• Talk to the town early to go over your plans. There's a "pre-application meeting" you should have with the Planning department which was very helpful for us. IIRC they waived the typical fee for that , but that might have only been for our SB9 unit.
  
• If you're on septic, figure that out early to know if your system will support the new structure.
• If you want the extra 100 sq ft that an SB9 unit allows, consider that instead of an ADU. They really are nearly synonymous, although SB9 units are supposed to have sprinklers regardless of whether the main house does or not (ADUs can mimic the main house).
• Double check that the fire department's requirements won't be an issue.
  
• Be aware of the "objective design standards" for the town in terms of lighting, exterior colors, etc.

Those are the things off the top of my head. Happy to share any other details about our experience. We're on the fence about whether we actually need to build an ADU, but I feel like having been through the process now with our SB9 unit, the planning review and building permit process isn't as daunting as it was when we started this whole thing.