I would issue the Building Permit but require a statement on the plans that no approval for foundation pour will be issued until the existing SS line is disconnected, the new rerouted SS line is built, inspected and operational, the old UE is vacated and (if needed) the new rerouted UE is recorded.
Please note that I'm an architect, not a building official. But I've seen chicken-vs.-egg scenarios where the developer first needs to have the BP in order to secure the construction loan which will then finance removal of the SS line.
Issuing the BP, but requiring evidence of easement vacation and SS line rerouting prior foundation pour, still helps ensure the ultimate public interest goal is met while giving the owner the flexibility they need.
On a separate note, here in one of the biggest cities in So Cal, we once got approval to build a HUD apartment complex that straddled an active public works storm drain line, with an active easement. (HUD is very particular about easements and ownership issues.)
The solution is that the line and easement passed underneath/through our 2-story high lobby, which was designed with a sacrificial floor slab and deepened footings to eliminate surcharge loads. We had extra large entry doors at either end of the lobby, and the lobby doors were not used as a means of egress. Point is, if the pipe ever needed replacing, we could open up the doors, drive in the equipment, sawcut the slab and dig as-needed to access and repair the storm drain line, all without evacuating the tenants.
