My partner left the UK after overstaying for 4 months. He is now banned from the Uk for a year. I'm wondering if he can reapply for his new visa before the year ban is complete, so that he can come back as soon as the ban has ended? Or will it be rejected as the year ban has not been completed?
The immigration rules state:
Previous breach of immigration laws grounds
9.8.1. An application for entry clearance or permission to enter must be refused if:
(a) the applicant has previously breached immigration laws; and
(b) the application is for entry clearance or permission to enter and it was made within the relevant time period in paragraph 9.8.7.
Paragraph 9.8.7 lists the different lengths of the bans.
So it's the date of application which counts, not the intended date of travel.
In any case, he'd better have a bullet-proof application in all other respects, as he has already shown not be trustworthy. Chances of a successful application are pretty low.
Note that rules are different for family members, though the details vary depending on the exact status of your relationship and other details.