We had a similar issue. Our application vanished into a void, "pending consideration" until the deadline approached, then we had an email to inform us that the council wouldn't be able to issue a decision within the statutory time frame and asking if we could possibly allow them longer to look at it.
Well, the alternative was for the application to be "treated as having been refused" and appeal, in our case to the devolved government which presumably has its own waiting list and might well have its own backlog, we allowed them this extension. I tried phoning to discuss it but "all officers working from home and can't be reached." Then they asked for another extension.
At this point we were ready to complain, but I thought I'd try a gentle approach first. I emailed the officer allocated to our case along the following lines:
"We are keen to make progress with construction of our extension, and have builders ready to start immediately. Consequently, we have been disappointed by the two letters we have now received requesting more time to consider the proposal, which means we are now hoping for a decision by as compared to the original statutory expiry date of .
We quite understand that the usual working processes of the planning department have likely been seriously disrupted by the ongoing pandemic and lockdown. Given that builders generally report high demand, I imagine that high numbers of applications may also be a factor influencing timescales for processing applications.
With all this in mind, please do not consider this email to be a complaint, but merely a polite enquiry: are you able to offer any indication as to whether the latest requested extension to provides a reasonable guide to when a decision can be expected, please?"
I received a reply to my email two days later, which said "I have just issued my report recommending approval of your application, and you should hear officially within 2-3 days". Result!