Date: 201908
Local time of day: 1801-2400
Location: SAN.Airport, US
Flight conditions: Mixed
Weather elements / visibility: Cloudy
Aircraft: Commercial Fixed Wing
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Final Approach
Airspace: Class C SAN
ATC / advisory: TRACON SCT
Reporter organization: Air Carrier
Reporter function: First Officer; Pilot Flying
Reporter qualification: Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP); Flight Crew Multiengine
Human factors: Time Pressure; Communication Breakdown; Human-Machine Interface
Second reporter function: Pilot Not Flying; Captain
Anomaly: Deviation - Altitude Crossing Restriction Not Met; Deviation - Altitude Overshoot; Deviation / Discrepancy - Procedural Published Material / Policy; Deviation / Discrepancy - Procedural Clearance
Detector: Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Provided Assistance; Flight Crew Returned To Clearance
Contributing factors: Human Factors; Procedure; Aircraft
Primary problem: Human Factors

Narrative: We were on descent into SAN; I was the Flying Pilot. The weather had been reported as scattered clouds 900 ft. I had briefed and setup for the Localizer 27 approach eventually transitioning to the visual. As we neared the airport I could see to what seemed to be a more solid low layer of clouds moving toward the airport which prompted some discussion between the Captain and myself. We both knew the RNP approach had lower minimums. As we approached KLOMN; the ATIS changed and the controller reported the ceiling as broken at 900 ft. This prompted me to once again consider switching to the RNP which the Captain promptly asked ATC for. Within seconds the Controller cleared us for the RNP approach. We were unable to get the approach loaded into the FMC as we received our clearance directly over KLOMN (IAF). I continued to fly the jet resorting to hand flying visually as I could clearly see the airport. The Captain got the correct way point in front of us and we continued the approach. At this point we realized we crossed CRSNR at 3;000 ft. rather than the 3;700 ft. per our IFR approach clearance. The rest of the approach went normally and we landed uneventfully. Where do I begin. Be careful what you ask for; you just might get it. I think the root cause for me was my complacency as SAN is a very familiar airport to me. I have flown both the localizer and RNP many times. In addition to this I could see the airport which caused me to transition to a visual approach while still on an IFR clearance. I didn't anticipate receiving such a quick approach clearance from the Controller and should have asked for a heading while we setup for the new approach. Trying to switch approaches at the last minute to obtain a lower minimums was a bad decision. I should have initially planned for the RNP approach or simply stuck with the localizer which would have worked out fine considering we could see the airport visually anyway. This was an excellent reminder for me to not be rushed; ask for; or accept a clearance you have not setup for or briefed ahead of time.


Second narrative: [Report narrative contained no additional information.]


Synopsis: Flight crew reported late approach clearance request resulted in crossing fix below published altitude.