How is an under-sink water filter properly installed in a Forest Hills co-op apartment?

The problem

A Forest Hills co-op owner in a Tudor-era building was completing a kitchen renovation and wanted an under-sink filtration system installed as part of the project. The building has older copper supply lines typical of pre-war Forest Hills construction, and the owner was concerned about the water quality given the age of the building's internal piping. A previous attempt by a handyman had left a saddle valve on the cold line that the building's plumber had flagged as non-compliant during a building inspection, the co-op board required a proper plumbing connection before the renovation could be closed out. The saddle valve had been installed at a copper elbow. Worst possible location since any vibration from the building system concentrates stress at the piercing mechanism.

What we did

We removed the non-compliant saddle valve and soldered a 1/4-turn brass tee fitting into the cold supply line under the sink, restoring the line to a clean, code-compliant configuration. We mounted the filter housing bracket to the cabinet interior, connected the inlet with a braided supply line from the new tee, and installed a dedicated brass ball valve between the tee and the filter for servicing. The filtered output was routed to the cold inlet of the existing faucet. We reinstalled the drain connections, which had been disturbed during the prior work, confirmed no leaks at all fittings, and labeled the shutoff valve and filter housing for the owner's reference at cartridge change intervals. Installed a 3M Aqua-Pure AP Easy Complete system. Whole-cartridge design that makes filter changes simple without tools. Labeled the dedicated shutoff and the filter housing with the cartridge change interval before leaving.