Does anyone know how long a municipal utility extension review actually takes start to finish

The 30 to 45 days is almost certainly the formal review clock only, meaning the window that starts once the city deems your application complete. What the website does not say is how long it takes to get to that acceptance point, which involves submitting, getting a deficiency notice, revising, resubmitting, and sometimes repeating that cycle two or three times before the clock officially starts. The six months your contacts quoted is probably a more honest picture of the full elapsed time from first submission to final determination, and that is before any appeals or engineering revision rounds triggered by reviewer comments.

The assumption doing the most work in your planning is whether the lot is even serviceable from the existing mains. Utility extension review scope and complexity varies a lot depending on whether the main is adjacent and adequately sized, or whether you would need to extend it a meaningful distance. If the nearest water or sewer main is undersized or set back from the lot, you may be looking at a capital contribution requirement or a cost-to-extend calculation that changes the economics entirely. That is a separate track from the review timeline and often surfaces mid-process.

The risk you have not mentioned: a lot split behind an existing duplex in a mid-size Ohio city may trigger a separate platting process on top of utility review. Those two tracks do not always run concurrently, and the platting step can add another review cycle with its own clock. Confirm with the city planning office whether an administrative lot split is available or whether a full subdivision plat is required. That distinction probably matters more to your timeline than the utility review window alone.

Before you model anything, I would call the city's engineering or public works department directly and ask them for the average time from initial submission to completeness determination, separate from the formal review period. Ask for a recent comparable project if they will share one.

What does the current zoning allow on that rear lot as a standalone parcel?

The six months number is right and the city website number is basically useless for planning. That 30-45 days doesn't touch the pre-application meeting, the engineer's preliminary report, or the two rounds of "your application is incomplete" letters you'll get before they ever start the clock.

What's the lot size and does it have any existing stub-outs already?

The city's published window is the formal clock after a complete application is accepted, and the pre-acceptance phase is where most of the calendar goes. In Ohio, a utility extension review often requires a preliminary feasibility letter from the utility district, a capacity confirmation, and sometimes a cost-share agreement before the city will even open the formal docket, and each of those steps has its own informal queue. Six months is realistic when you add the back-and-forth to get the application deemed complete, the formal 30 to 45 days, and then any comment response cycles. The number that actually decides your planning timeline is how far the nearest water and sewer stub sits from the lot line, because a short extension with adequate capacity in the main can compress everything, while an undersized main triggers a capital improvement conversation that can stretch well past a year.

What does the city's GIS or public utility map show for the main size and the distance to the nearest stub on that block?

The distinction you're drawing is exactly right, and it's the one that costs people a full season. The 30 to 45 days is the clock they run after they stamp the application complete, but getting to "complete" in a mid-size Ohio city can take three or four rounds of corrections that each restart a two or three week internal comment period nobody tells you about upfront. I burned about four months on a pre-application ping-pong in Wrocław on a much simpler utility matter, and that city at least admits the informal phase exists. What I'd actually want to know before I budgeted any timeline: does that lot have frontage on a street with existing water and sewer mains, or is it genuinely a main extension situation, because those are two completely different animals and the six-month number probably applies to the second one.

Ohio municipalities vary a lot on this, Columbus suburban rings move differently than Youngstown or Dayton where staff is thin and one engineer is reviewing everything.

Ask specifically whether they require a pre-application meeting before they will accept anything, because that meeting alone can be scheduled six to eight weeks out in some offices, and that is before the 30 to 45 day clock even starts.

What city is it, roughly?

Call the city engineer's office directly and ask what their current backlog is.

The number on the city website almost certainly counts only the formal review period after they stamp an application complete, and "complete" is doing a lot of work in that sentence. Before that clock starts, you typically go through a pre-application meeting, revisions to the utility extension study, possible coordination with the water or sewer district if it is a separate authority from the city (common in Ohio outside the largest cities), and then a completeness review that can bounce the application back once or twice. That pre-acceptance phase is where most of the six months lives. The related piece that changes the picture here: if the lot split itself requires a replat rather than just an administrative lot division, the plat review runs on its own parallel track with its own referral to the county, and the two timelines do not always sync, so you can clear utility review and then wait on the plat, or vice versa. Before you price any holding cost into a feasibility model, the most useful thing to ask the city engineer's office is not how long review takes but how many submission cycles their last three utility extension applications required before acceptance, because that tells you the real number.

What is the size of the vacant lot, and do you know whether the city's zoning would allow a single additional unit by right or whether you'd be looking at a variance?