If exceptional circumstances are affecting your ability to work, in the final 3 months before your submission deadline you may be permitted to extend your submission deadline, but only in proportion to the documented impact on your work. You should be planning to submit considerably before the deadline to allow for less exceptional but still unpredictable events that might affect your work.
There is no guarantee that you will be able to extend your programme in this way, especially if the circumstances could have been remedied with a leave of absence at an earlier stage. Multiple extensions exceeding a total period of two years will not normally be granted.
Applications for extensions should be made before your current submission deadline. If you need to apply for an extension to your submission deadline, you should speak to your supervisor in the first instance. The length of extension should be justified by the circumstances and supported by the departmental Graduate Chair or the Chair or Deputy Chairs of the Departmental Education Committee.