1, Do ADUs generally have separate utility meters?
A lot of jurisdictions won't allow a separate meter because then you pretty much have a duplex. Getting a new meter from PGE takes a very long time and is usually not worth the cost. So most ADUS I have seen have a combined utility meter and then a submeter on the ADU.
Does it worth the effort to have separate meters?
I don’t think so. But having one meter makes it a little more tricky with tenant if you want to charge for utilities.
2, Anybody has any experience building ADU with existing tenant on the property? I would imagine it will create some inconvenience to the the tenant. What are some of the steps I can take to minimize disputes/complaints from existing tenant?
Yes I have Guess what happened? They got very upset about the noise (they had just had a new born baby) and they demanded to break their lease early which I let them Make sure your tenants are on board and be very clear that there will be a lot of noise
3, I could get HELOC from either my primary home or my rental property to finance the ADU. Which is the better approach?
I think either is fine, but consult your CPA about tax implications.