In November and December of 2025, we filed two ADU permits with the City of Tempe. Same drafter — me. Same project size — 720 square feet, detached unit, one bedroom, one bathroom. Same partner electrical engineer. The clients lived six blocks apart in the McClintock-Broadway corridor.
The first one cleared in 28 days. The second one took 84.
When the second client called to ask why his was taking so long, I had to actually explain it. Both ADUs were code-compliant. Both sets followed the same format. The difference wasn't the drafting — it was the lot. Specifically, the zoning designation of the parent parcel.
R1-6 vs. R1-8: thirteen feet that change everything
Tempe has two main single-family zoning districts in the McClintock-Broadway area. R1-6 (6,000-square-foot minimum lot) and R1-8 (8,000-square-foot minimum lot). Both allow ADUs under the 2024 statewide statute. But they don't allow them on the same terms.
- R1-6 lots: ADU rear setback is 5 feet, side setback is 3 feet. Height max is 22 feet. No second-story height restriction beyond that. Lot coverage cap is 50%.
- R1-8 lots: ADU rear setback is 10 feet, side setback is 5 feet. Height max is 17 feet for any portion within 10 feet of a property line. Lot coverage cap is 40%.
My first client (28 days) had a 6,200-square-foot R1-6 lot. The proposed ADU sat 8 feet from the rear property line and 5 feet from the side. Building height was 19 feet at the ridge. Lot coverage including the new structure: 47%. Every dimension was inside the R1-6 envelope with margin to spare. Plan-check picked it up, ran the setback verification once, and signed off.
My second client (84 days) had an 8,400-square-foot R1-8 lot. The proposed ADU sat 11 feet from the rear and 6 feet from the side, building height 19 feet at the ridge. Setbacks were fine. But the height-near-property-line rule kicked in: any portion of the structure within 10 feet of the side property line had to be 17 feet or shorter. The ridge was 6 feet from the side. The ridge was 19 feet tall. The ridge violated the envelope.
The fix was a roof redesign — drop the ridge, change the roof slope, redo the elevations. We did it. Plan-check returned the set with two more comments after that, both about lot coverage now that the new roof had changed the soffit overhang and tipped us over 40%. Each round of comments was 12-18 days in the queue.
The City of Tempe doesn't advertise the difference
Here's the thing that bothers me. Tempe's ADU information page on the city website mentions setbacks but does not call out the height-envelope difference between R1-6 and R1-8. A homeowner reading that page would conclude their lot is ADU-eligible — true — without realizing the design constraints can vary by 30% in usable building volume.
I called the Tempe Planning Department in February to ask if the page was being updated. The answer was "it's on the list." I have not seen it updated as of late April.
How to know which you've got
Pull your parcel up on the Maricopa County Assessor site. The legal description includes the zoning code. If it says R1-6 (or LR3), you have the more permissive envelope. If it says R1-8 (or LR2), the height-near-property-line rule is going to bite you.
If you have an R1-8 lot and are committed to a 19-foot-plus ridge, plan for a re-design after first plan-check. That's at least one extra cycle. We design our R1-8 ADUs at 16'9" ridge by default, which clears the envelope on every Tempe lot we've looked at.
If your neighbor's ADU cleared fast, look up their lot's zoning. If it's R1-6 and yours is R1-8, you're not running the same race.
We did one last R1-8 ADU in March. Designed to 16'9" ridge from day one. Cleared in 31 days, first review. Same building department, same season, same code. Different envelope strategy.
The state ADU law that everyone got excited about in 2024 doesn't override local zoning envelope rules. It opens up where you can put an ADU, but the city still controls how big it can be. Different problem.