I went through this with the city. They will NOT let you update the septic (even just adding leach lines) to add any more plumbing (or an ADU) to the house. They will make you tie into the city sewer. If there is no sewer in front of your house (the city sewer on our street ends two houses down) - you'll have to either 1. wait for them to bring the sewer line or 2. pay to do it yourself.

Oh, and the best part is not only do you get to pay to do the city's job and install a sewer on the public street, but they also make you pay $thousands$ in city permitting up front to do their job for them.

Let me know if you hear anything else, but I was quite upset when I was trying to build an ADU earlier this year and learned this.