If you have a lease agreement,the ISTAT indexation mechanism is essential to understand. Simply put, it is the annual adjustment of the rent to offset the loss of purchasing power caused by inflation. But be careful: if your lease is subject to the flat-rate tax regime, you can skip directly to the last paragraph, because this adjustment does not apply. For everyone else, this mechanism allows the landlord to adjust the rent based on the consumer price index (FOI index) published by ISTAT, as provided for in Article 32 of Law 392/1978 (Art. 32) and subsequent amendments. The application varies: typically, it is 75% of the change for open-market leases (4+4) and can reach 100% for commercial leases or negotiated rents. In this guide, we explain step by step how to calculate it, when to notify tenants, and how to handle special cases.

What is the ISTAT adjustment and how does it work?

The ISTAT adjustment is the mechanism that allows the landlord to adjust the rent in line with the cost of living. It is not an arbitrary increase, but a mathematical adjustment based on public and official data:the FOI index (Consumer Price Index for Blue-Collar and White-Collar Workers), which the National Institute of Statistics (ISTAT) publishes every month.

In order to applythe ISTAT rent index, the lease must include a specific clause providing for this. Without this clause, the rent remains fixed.

The main rules are:

• Open-market leases (4+4): For standard residential leases, rent increases are capped at 75% of the change in the ISTAT index.
• Leases for other purposes (commercial) and agreed rent: In these cases, up to 100% of the index change may be applied.

Important: The golden rule is that rent adjustment is a right of the landlord, but not an obligation. To make it effective, it must be formally requested from the tenant each year.

How to Calculate the ISTAT Adjustment Step by Step

Calculating the ISTAT adjustment may seem complicated, but it boils down to a simple formula. Here’s how to do it, step by step.

1. Find the Percentage Change in the FOI Index
The key figure is the annual percentage change in the FOI index. You can find it directly on the ISTAT website or, for your convenience, in the table we have prepared below. It is essential to use the change between the month the contract was signed (or the last adjustment) and the same month of the current year. A common mistake is to use the old index with a 2010 base instead of the current one (2015 base).

2. Apply the formula
. The formula for calculating the increase is:

Current Annual Rent × (FOI Percentage Change × Adjustment Percentage)

The "Adjustment Percentage" is the one specified in your contract (usually 75% or 100%).

3. Practical Example
Let's see how it works with a real-world example:

• Monthly rent: €600 (or €7,200 per year).
• Contract type: Open-ended lease (4+4) with a 75% rent adjustment.
• Reference month: February.
• Annual ISTAT change (February over February): Let’s assume it is +1.1%.

Now let's apply the formula:

1. Calculation of the actual adjustment: 1.1% (ISTAT change) × 75% (contract clause) = 0.825%. This is the actual percentage to be applied .
2. Calculation of the annual increase: €7,200 × 0.825% = €59.40.
3. Calculation of the monthly increase: €59.40 / 12 = €4.95.
4. New monthly fee: €600 + €4.95 = €604.95.

Starting with the first billing cycle following notification, the new monthly fee will be €604.95. A similar process, based on specific indices, is also used to calculate the adjustment of severance pay.

Table: Changes in the FOI Index (last 24 months)

To make things easier for you, here is a table showing the latest annual percentage changes in the FOI index, excluding tobacco. Find the reference month for your contract and use the corresponding value.

Source: ISTAT. The data are updated periodically to reflect the latest releases. For official and complete data, see the analysis of consumer prices.

If you manage multiple contracts, organizing your data in a spreadsheet is essential. You can start with a practical example of an Excel table to track deadlines and calculations.

When and how to communicate the adjustment

Have you calculated the new rent? Great. Now you need to report it correctly. A mistake here could cost you months of adjustments.

The law requires a formal notification that provides proof of receipt. The only valid methods are:

• Registered mail with return receipt: The safest and most traditional method.
• Certified Email (PEC): If both parties (landlord and tenant) have a PEC, it has the same legal standing as a certified letter.

The most important rule: no retroactive application. The rent increase applies only starting from the month following the one in which the tenant receives your notice. If the annual renewal date is in June and the tenant receives your letter on June 2, the increase takes effect starting with the July payment. Many landlords miss out on months of rent adjustments because they don’t send the notice far enough in advance.

To keep track of your communications, you can create a log, perhaps using a spreadsheet as explained in our guide on how to create a chart in Excel.

Edge Cases You Need to Know About

Here are the answers to the most frequently asked questionsabout ISTAT’s rent index.

• What happens if the FOI index is negative (deflation)? Technically, the rent should decrease. However, most leases include a clause specifying that the adjustment applies only in the event of a positive change, to protect the landlord. Check your lease.
• Does the adjustment apply from the first year? No. The first reassessment can only be requested after the first 12 months have elapsed from the contract’s effective date.
• I forgot to request the raise for two years—can I claim the back pay? No, that’s not possible. The right to a raise expires on an annual basis. If you don’t request it, you lose that right for that period. You can only request the raise for the current year, which will take effect the month following your request. The case law on this point is very clear.

The exception that brings everything to a standstill: the flat-rate tax

Let’s get to the point that makes life easier for many: the flat-rate tax. The rule is simple and absolute: if, as a landlord, you’ve opted for the flat-rate tax regime, you’ve waived any right to adjust the rent. Period.

This means thatthe ISTAT rent index does not apply for the entire duration of the option. It is an agreement with the government: in exchange for tax relief, you agree not to raise the rent.

If your lease is under the flat-rate tax scheme, you can completely disregard calculations, indices, and notices. The ISTAT adjustment simply does not apply to you. A request for a rent increase would be unlawful, and the tenant would have every right to refuse it.

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