The first thing to do is to record the damages. Take a written note of all damages and it is also advisable to take photographic evidence. A digital camera is ideal for this purpose.
Establish the costs of the damage
If possible, it is best to get professional estimates of the cost of any repairs. If the matter comes before the Private Residential Tenancies Board (PRTB) it will not be sufficient to have estimated the costs yourself. It is also not allowable to put a cost on your own time for repairs and to deduct this cost from the deposit or to claim as an expense against tax. Keep copies of all receipts and invoices for repair work carried out.
Put it in writing
The tenant should be informed in writing of the reasons for withholding all or part of the deposit and the cost of any repairs.
See also dealing with wear and tear