PC12 pilot reported receiving altitude warning during descent. Pilot corrected altitude and continued uneventfully.

Narrative

The decision to return to base was made enroute from ZZZ to ZZZ1 due to Un forecast weather conditions to ZZZ1 airport. We had just climbed through some cumulus clouds and picked up some light rime ice at 5;000ft. We cleared the tops of the clouds at FL120 and cruised at FL150. Center advised of some moderate showers enroute to ZZZZZ the IAF for the Rnav X into ZZZ1. ZZZ [Center] gave me direct to ZZZZZ1 IAF to avoid some of the weather for a straight in for the RNAV Y RWY XX. The decision to turn back to base was based on southerly winds on the approach with building cumulus clouds and ice conditions with a circle to land at ZZZ1. I decided that the swiss cheese model was starting to line up and decided to return to base.I requested the turn back through ZZZ [Center] and was given direct back to ZZZ. At this point I selected heading mode for an initial 180 degree turn to head back as I programmed the FMS back to base. I was given a pilots discretion to FL10. I maintained FL150 until I finished loading the FMS and reviewed the weather enroute back to the ZZZ. As I started my descent I was asked about the weather and notams at ZZZ and asked for deviations north of course to avoid weather build ups. Passing through 11;000 ZZZ gave me a another pilots discretion to 5;000. I set the altitude selection to 5;000 and continued my descent. On my way down I disconnected the AP (Auto Pilot) to avoid some weather and was VMC by 6;000. I called ZZZ [Center] to cancel my IFR but stepped on by another Transmisson. As I went to call back ZZZ center gave me an altitude warning and I realized I was at 4;300 ft. The MVA for that sector is 5;000 I made a correction to 5;000 and called to cancel my IFR flight plan and navigated back to base VFR. Spring time weather in the area changes very rapidly and decisions have to be made promptly. Looking back at this situation I would have turned the AP back on so I could focus on other tasks at hand. Turn backs can get very busy and always an opportunity for several distractions. Single pilot environment the AP is crucial and needs to be turned on to reduce the workload. ZZZ center was instrumental in assisting me with this transport and I appreciated their prompt info about changing conditions. Looking forward I would need to turn on the AP and call for cancellation for VFR a bit earlier.