Tier 2 extension maintenance funds

[K_Bilo]

I am applying to extend my tier 2 (general) visa from inside the UK. My current visa expires in 90 days.
Do I need to give documents proving I have £945 in my bank for 90 days? If so, I can't do this in time for my current visa to expire. I make well over £945 a month and can prove this with pay slips, however I can't prove that £945 has been there for 90 consecutive days.
If this is the case, how does my sponsor 'certify my maintenance?'. Will they have to apply for a new COS number?

If this isn't possible, can I stay in the UK if my current visa expires while my new visa is being processed?

Thank you

[iworker]

if you are applying for extension with same employer, then u dont need to certify maintenance.
If it is new a employer, ask him to certify the maintenance while issuing the cos.

[K_Bilo]

if you are applying for extension with same employer, then u dont need to certify maintenance.
If it is new a employer, ask him to certify the maintenance while issuing the cos.

Thank you for the reply. I am with the same employer, but when I am filling out my application form it asks 'is your sponsor certifying your maintenance'. They aren't, so I have clicked 'no'. Now it is asking me for proof of the £945. I'm not sure what to then include in my application

[Akorotaeva]

As far as I know, if you have existing Tier 2 Leave to Remain, you don't need to certify maintenance. I am changing employers and although they certified maintenance, my application states the following:

Information you do not have to provide
You do not need to prove you meet the required level of English because you have current leave in Tier 2
(General) leave to remain, or had leave in the past
You do not need to prove maintenance because you have current leave in Tier 2 (General) leave to remain