You can change your company’s year end (also known as its ‘accounting reference date’) to make your company’s financial year run for more or less than 12 months.
You can only do this for your company’s current financial year or the one immediately before it.
Changing your company’s year end will also change your deadline for filing accounts, unless you’re lengthening your company’s first financial year.
https://www.gov.uk/change-your-companys-year-end
If you shorten your year end by 1 day, you will get an extra 3 months from the date of filing the notice to shorten, so for example, if your year end was 31st March and your changed it to 30th March, due to file on 31st December and filed a notice on the 31st December, you could get until the 31st March to file
The rules are in http://www.legislation.gov.uk/ukpga/2006/46/section/442
If the relevant accounting reference period is treated as shortened by virtue of a notice given by the company under section 392 (alteration of accounting reference date), the period is—
(a)that applicable in accordance with the above provisions, or
(b)three months from the date of the notice under that section,
whichever last expires.
