Is Spring the last semester (in CC) to transfer courses to a UC? Can I take transferrable courses in the summer?
Yes, with some exceptions (i.e. UCB Psychology’s new Calculus requirement can be completed over the summer).
Generally, anything you take over the summer will not count towards the UC’s - and if you do poorly, you will be rescinded.
If you are asking about whether the courses you are taking to meet UC requirements such as the 7-course requiremet, these must be completed by spring. However if you are asking about just taking an extra class to earn additional credits, I don’t see anything on the UC websites that says you can’t do that.