Does an ADU need its own water and sewer connection? Post Falls' answer changed.

By Russell Mann — Post Falls, Idaho

Back in July I wrote that my ADU would have to share the main house's water and sewer, and that sharing them is the reason the city charges no capitalization fee for it. I had it by email from Tisha Gallop, a permit technician for the city, and I put it on the tracker and in two posts as a fact.

In August she told me it isn't a rule. I can separate the ADU's water and sewer if I want to. Here's what that costs, and why I'm connecting to the house anyway.

What Tisha corrected

On July 7 I asked her a short version of the question and got a short answer back. An ADU "will need to be" connected to the house for water and sewer, so there's no separate capitalization fee. It read like a requirement, so I wrote it down as one.

I asked again on August 5. This time she checked with the building, engineering and planning departments before she wrote back. Her words: "you can but are not required to hook up to any of the house utilities."

In July I thought I had no choice about it. Turns out I do.

What separating costs

If I put in the ADU's own water and sewer, here's the bill Tisha laid out:

Water capitalization fee plus a new meter: about $3,750.
Sewer capitalization fee: $7,621.
Planning review on top of both, decided case by case on lot size, orientation and zoning.

Call it $11,000, and it still requires city approval.

Water and sewer go together here. Separate one and you have to separate the other. I haven't seen that written down anywhere.

The Avista wiring

The way Avista wants to run gas and electric to my ADU is one line into the property feeding a shared meter pack. I asked Tisha whether that counts as separating the utilities. It does. Any setup where the ADU isn't fully connected to the house goes to Planning for review.

She also told me something I hadn't asked about. Even if Avista is fine with the separate lines and meters, the city "might not issue permits for those new lines or meters to be connected to the ADU," and I should check with Planning before I set anything up with Avista and pay for it.

So I'm not scheduling any Avista work until Planning has looked at the plan. The $1,000 to $2,000 I wrote about for the electric trench assumes the connection gets approved.

The cheaper way

There's a middle option I didn't know about until I asked. Post Falls will let me put a private meter on the line after the city's meter. A deduct meter. The city still reads one meter and sends one bill, but I can read the second one myself, see what the ADU used, and back that number out. No second tap into the main, no sewer capitalization fee.

That covers most of what I wanted a separate meter for. If somebody rents the ADU I want to know what their water costs. I don't need the city to run a separate account for that.

I still don't have the price to install one.

Who pays for growth

One thing here I keep turning over. The capitalization fee is the price of drawing more water and sewer capacity than the lot already pays for. Connect to the house and I'm not drawing any more, so there's nothing to pay. Put in my own service and I am, so the sewer side alone runs $7,621.

That's the "growth should pay for itself" argument working the way people ask it to, at least for water and sewer. Roads and school seats are the part people usually mean.

The follow-up email

I wrote Tisha back with a few more questions. The answer I got on August 10 settled one of them and left me more confused about the rest.

One thing got settled: the ADU gets its own address. It's the house's address with "Unit A" on the end.

Still tangled up:

The city might not permit separate gas and electric lines to the ADU even if Avista is willing to install them. So I still don't know whether separating them is allowed.
I don't know what a deduct meter costs, or whether the city has to inspect and sign off on one.
I've asked twice now whether a shared connection still gets the ADU its own city water bill. Both times the reply was about something else.

I've sent Tisha another email. Shorter and more direct this time, because I think my first round of questions was too tangled to answer cleanly. I'll post what she says.

Everything above is on the project tracker with the running total. If you've separated an ADU's water or sewer in Post Falls, or you got a different number than mine, I'd like to hear it. Leave a comment below or use the contact form.

Sources: emails from Tisha Gallop, Permit Technician, City of Post Falls, on August 5 and August 10, 2026, each after she consulted the Building, Engineering and Planning departments. Fee figures are hers; the sewer number matches the city's published fee schedule and the water number appears to bundle the meter.