Yes, it's a good idea to put it in the lease. Remember that the deposit must be put in an interest-bearing account for the duration of the lease and given back to your tenant, plus the interest it has earned, when the tenant moves out.
If, however, your tenant still owes you money on moving out, or if the property has been damaged beyond normal wear and tear, you can use the deposit to pay for repairs or to cover the money owed to you.
It is common for the landlord to require one month’s rent or a double deposit (2 month’s rent). The landlord could even require a triple deposit if the tenant's credit report is considered risky.
However once the amount of the deposit has been agreed, the landlord cannot demand a bigger deposit during the term of the lease – unless the tenant agrees, or the lease agreement makes provision for a top-up when the rent increases
Practically, the landlord should not hand over the keys to the property until the agreed deposit and first month’s rent has been paid (and cleared in the case of a cheque).
In terms of the Rental Housing Act, if the landlord holds the deposit, he / she must invest the deposit in an interest bearing account with a minimum rate of interest applicable to a savings account. The landlord cannot contract out of this legal obligation.
Tenants are entitled to request written proof of the interest earned and if requested, the landlord is obliged to provide such proof.
If the deposit is held by the estate agent, this is regulated by the Estate Agents Affairs Act – read your lease agreement – some lease agreements provide that no interest is paid to the tenant and in this case, the estate agent does not need to refund the deposit with interest.