To file a lawsuit for the determination of rent, there must be a written or oral lease agreement. Without a written or oral lease agreement, a lawsuit for the determination of rent cannot be filed against natural or legal persons. As in all lawsuits, the claimant must have a legal interest in filing the case. The lawsuit may be filed by either the tenant or the landlord. The landlord may file the lawsuit to increase a low rent, while the tenant may file it to reduce an excessively high rent.

Each co-owner may independently file a rent determination lawsuit; in this case, the determination is requested only for the rent corresponding to their own share. All co-owners may file the lawsuit jointly, or the other co-owners may later join a lawsuit filed by one of them.

In cases of joint ownership, all owners must file the lawsuit together, or the other owners must join the filed lawsuit or grant written authorization.

If the lease agreement between the parties contains a provision regarding the determination of the rent for subsequent rental periods, the rent shall be determined in accordance with this agreement. However, such determination may not exceed the twelve-month average of the Consumer Price Index (CPI).
If there is no agreement between the parties, a rent determination lawsuit may be filed for each rental period, provided that the initial one-year period has elapsed. When determining the rent, the judge shall ensure that it does not exceed the twelve-month CPI average and shall consider principles of equity.

Regardless of whether there is an agreement regarding future rent, a rent determination lawsuit may be filed for lease agreements lasting longer than five years or renewed after five years. This applies for each five-year period. The judge shall decide based on the twelve-month CPI average, comparable rental values, and equity.
If the rent is denominated in foreign currency, it cannot be changed before five years. After five years, changes in the value of the foreign currency shall be taken into account in determining the rent.

A rent determination lawsuit shall be filed before the Civil Court of Peace (Sulh Hukuk Mahkemesi) at the defendant’s place of residence or where the leased immovable property is located. However, if the parties to the contract are merchants or public legal entities, the court designated in the contract shall have jurisdiction pursuant to Article 17 of the Code of Civil Procedure.

In rent determination lawsuits, a proportional fee is charged based on the monthly gross rent difference. There are also court decisions where the fee is calculated based on the annual gross rent difference for leases with annual rent payments.

A rent determination lawsuit cannot be filed as a partial claim or an indefinite claim. The requested rent amount must be clearly stated in full in the petition. An amendment request cannot be made during the proceedings to increase the claimed rent amount.

In determining the new rent amount, expert reports, comparable lease agreements, and comparable market rental values are of importance. The lease agreement between the parties constitutes evidence. If there is no written agreement, bank receipts and electricity, water, and natural gas subscriptions serve as evidence.
Factors such as the location, surroundings, size, characteristics of the property, lease start date, and duration are examined. The judge decides based on principles of fairness and equity. In the case of an existing (long-term) tenant, the judge may apply a certain discount.

With the regulation introduced, as of 1 September 2023, applying to a mediator has become a prerequisite for filing rent determination lawsuits. Lawsuits filed without prior mediation are dismissed on procedural grounds.

Sending a notice is not a condition for filing the lawsuit; however, it is necessary for the rent determined as a result of the lawsuit to be effective as of the beginning of the new rental period.
Provided that the lawsuit is filed at least thirty days before the start of the new rental period or that the landlord gives written notice to the tenant within this period stating that the rent will be increased, and the lawsuit is filed by the end of the following new rental period, the determined rent shall bind the tenant as of the beginning of the new rental period.

Judgments rendered in rent determination lawsuits cannot be enforced before they become final. Claims stated in the judgment, such as litigation costs and attorney’s fees, also cannot be enforced before the decision becomes final.

The rent increase rate may not exceed the twelve-month average of the Consumer Price Index (CPI). Between 01.07.2023 and 01.07.2024, a maximum increase rate of 25% was applied to residential rents. This regulation ended on 01.07.2024.