A Bonanza pilot reported a temporary loss of control due to unexpected turbulence that resulted in an altitude excursion.

Narrative

Departing ZZZ I was given a squawk; vector and clearance to filed altitude (7000 feet); but no route clearance or destination.During the climb I was in IMC and just leveling at 7000 feet in a what turned out to be a few miles of very turbulent build ups with the Autopilot in alt hold mode. At the point of possible deviation; the Autopilot was not able to hold 7000; and ascended rapidly. Disengaging the Autopilot required a lot of pressure to arrest the ascent. I did exceed the 300 foot exceed the buffer during the excursion; likely by less than 100 feet as best I can tell from the website data. I did not think it was prudent to approach a zero g pitch over to return to 7000 feet; but corrected in as timely a manner as I felt prudent without excessive nose down pitch.Had I known the turbulence was going to be as strong as experienced I should have advised unable to hold altitude. It was like a sudden elevator ride up. The winds aloft were approaching 50 kts. and the turbulence in clouds made control challenging. Both for me and the autopilot. While it was turbulent when not in cloud; it was nothing like the turbulence inside the buildups.For the reminder of the flight I requested and was cleared to 9000 feet; above the build ups; and a smooth flight.Certainly requesting heading deviations might have averted this; but there were a lot of them; and in this case I didn't see the one that produced the event prior to flying into it.The chain of events was as leveling at 7000 feet in IMC and engaging the altitude hold; I flew into a very turbulent but small area of build ups. Both ATC and I noted the excursion at the same time.Asking for heading deviations would have likely avoided some of the taller buildups; but in this case I was in them before I could visually evaluate what was coming next.