In addition to a certificate or opinion of compliance the mortgage company will look for proof that you have paid the council development fees before the last drawdown can be made. In ROI some local authorities will charge for extensions over 40sqm, others will not. The amount to be paid will have been outlined on your planning permission; proof of receipt of payment is what the banks are after.

You will also need a house valuation, done by a real estate agent appointed by the bank, to ensure the house is at least worth what they expect. You will then have the self-build insurance transferred to house insurance on that basis, as house insurance is based on the cost of rebuilding the house. 

Remember that a final certificate from your certifier effectively states that the construction contract has been fully completed and has the effect of releasing all remaining money due to the contractor, including any retention amount. Before it can be issued therefore, all patent defects must have been remedied, all adjustments to the contract sum agreed and all claims settled.