Replace the tank once, with the scope defined before anyone digs.

ZE does not quote prices, because the range is wide and the reasons are specific to the site. The big drivers: whether the tank is underground or aboveground (excavation, removal, disposal and backfill are a large share of a UST job); tank size, listing and construction; how much of the piping, sumps and dispensers come out with it; what is found in the ground (contaminated soil changes the job); electrical, paving, canopy and site work disturbed by the excavation; permit fees, PE sealing, testing and inspection; how long the facility can be down, and whether the work has to be phased to keep it open. A design-first scope lets you see these before contractors price them, and lets the contractors price the same job.

There is no universal age limit. Replacement is usually forced by a failed tightness or sump test, corrosion or structural findings, a single-wall tank or piping in a jurisdiction that requires secondary containment on replacement, an AHJ order or a violation that cannot be corrected in place, or an operational need for more capacity. Suffolk County keeps existing USTs on a testing schedule under Article 12, and a tank that keeps failing is a replacement candidate even without an order. ZE gives you the evidence and the recommendation.

Often, and it is common for #2 fuel oil and generator diesel. The AST has to be sited to fire code separation distances (a UL 2085 protected tank frequently makes a tight site work), the fill, vent, overfill and containment have to meet the county's AST requirements, and the UST has to be properly closed and deregistered. In Suffolk County both the new AST and the closure go through SCDHS. The design covers both halves.

Usually the sump, sometimes the fittings, occasionally more. A sound sump with a failed penetration boot can often be corrected with new entry fittings. A cracked sump, a sump with no test boundary, or one the AHJ will not accept for repair gets replaced. If the piping or the tank also shows problems in the same test round, the conversation changes. ZE reads the test report and the sump before recommending either.

Not automatically, but modifications trigger current requirements for the parts you touch, and some AHJs treat a substantial modification as a new installation for review purposes. In Suffolk County, altering an existing facility requires a permit to construct, and the reviewed drawings will be held to current Article 12 standards for the work shown. ZE identifies what the scope triggers before you commit to it, so there are no surprise upgrades at inspection.