You hear about this all the time, but I have never met anyone that has done it as there are always issues once you get into the details. The key themes I see when people evaluate doing this is that: 1) There would be so much time finding existing lines and retrofitting that just digging a new line is more time effective and less labor (ultimately cheaper); 2) The trunk line you could use does not slope properly to the connection point required by the city to complete the tap; 3) The trunk line is not the correct size to meet city code.
What I can tell you about tap fees are that they swing wildly between municipalities. Especially these little towns - they have a tap fee but then they hook on all sorts of extra fees depending on the amount of “non-standard” work they have to perform, like road bores. One time I was having a tap completed and it took the city three (3) days to just find where the existing sewer lines were located - and guess what - it was under the middle of the road. Guess who got billed for that? This guy right here. Then add your impact fee which is some arbitrary capital improvement number the city uses to say they will use towards capacity to their WWTP.
I’d really like to hear about a success story here!