The deposit:

This is a sum of money you pay to the lessor at the signing of the contract. This payment is to cover any deficiencies to your obligations you commit to housing for the duration of the lease (degradation, non-payment of rent and / or charges).

The security deposit is required?
Regarding the locations of empty spaces for residential use (Law of 6 July 1989) the deposit is not mandatory. You must pay your landlord only if its existence is mentioned in a clause of the lease (which is the case in most contracts). The deposit amount must be explicitly noted in the contract.
If your contract provides for payment of rent in advance and by quarter (ie at the end of each quarter you pay your landlord the amount of the next three months). The landlord can not require you to pay a security deposit (article 22 Act of 6 July 1989). However if you as permitted by law (Article 7 of Act 6 July 1989) you require, during the lease, a monthly payment of rent, the landlord may ask you to pay a security deposit.

Its amount and its revision:
The security deposit may not exceed two months rent off charges (Article 22 Act of 6 July 1989). If the owner has the right to collect the deposit, it can not, however, revise the amount in the lease or the renewal of the lease. (more…)

 The evidence required for the lease:

The lessor must ensure the creditworthiness of the tenant to reduce the risk of unpaid rents. The most common practice is to seek income equivalent to three times the rent and expenses. But the current tension in the rental market with rising rents that some landlords require a monthly income equal to four times the amount of rent excluding.

The landlord may ask you several documents:

your last three pay slips;
your certificate of employment;
your employment contract;
the latest receipts of your previous home;
your tax return on income;
your notice in particular for professionals and company managers;
a deposit of one month’s rent excluding returnable at the end of the lease by the state of play.

Note that if the future lease must be signed by partners or persons who entered into a PACS, the owner may apply to each signatory to provide these documents.

candidate to lease the following documents:

a photograph of identity;
social insurance card;
a copy of bank account or post office;
a certificate of good performance of bank or postal account. (more…)