There are two filing deadlines for annual accounts because you have to prepare a set for Companies House and HMRC.
The deadline for submitting accounts to Companies House is 9 months after the end of your company’s financial year. Your financial year ends on your accounting reference date (ARD), which is the anniversary of the last day of the month your company was formed - e.g. if your company was formed on 1st November 2015, your ARD will be 30th November 2016.
The deadline for submitting accounts to HMRC is 12 months after the end of your Corporation Tax accounting period. Your accounts for HMRC must be included with your Company Tax Return.