What a Rent Increase Is Allowed to Be

There is no such thing as the allowed increase

The question sounds like it has one answer and it has at least six, differing not only in level but in kind. In one market the increase is a formula applied to a published index and the landlord has no discretion at all. In another the landlord may raise the rent towards a locally surveyed comparable level, but no faster than a fixed percentage over three years. In a third the amount is whatever the lease says, subject to giving notice. Any article, calculator or forum post that quotes a single percentage without naming the jurisdiction is describing one of these and implying all of them.

Two structural distinctions do the most work. The first is whether the increase is automatic or must be claimed: in several of these systems a rent revision does not happen unless the lease provides for it and the landlord actually asks, in time and in the correct form. The second is whether the ceiling is a rate or a level. A rate ceiling — an index, a percentage cap — limits how fast the rent may move and says nothing about where it ends up. A level ceiling — a comparable local rent, a reference table — limits where the rent may end up and says nothing about how it gets there. Germany applies both at once, and much of the confusion in that market comes from readers who have grasped one of the two.

The five that index, and what to

France uses the indice de référence des loyers, a quarterly series published by INSEE. Where the lease provides for revision, the calculation is fixed by the 1989 tenancy act: the new rent equals the current rent multiplied by the index for the reference quarter and divided by the index for the same quarter a year earlier. The landlord has no discretion over the amount, only over whether to apply it, and the right to claim a revision lapses if it is not exercised within a year. Portugal is structurally the simplest of all: the statistics institute computes a single annual coefficient from the twelve-month change in consumer prices excluding housing, using data available at 31 August, and it is published by notice in the official gazette before the end of October for the following year. For 2026 that coefficient is 1.0224; for 2025 it was 1.0216.

Spain rebuilt its mechanism recently and the design is worth noting. The 2023 housing act instructed the national statistics institute to define, before the end of 2024, a reference index to serve as the limit on annual updates. The resulting index, in force from 1 January 2025 and published monthly to two decimals, is defined as the minimum of three measures: the annual rate of the consumer price index, the annual rate of core inflation, and an adjusted average annual rate. Taking a minimum of three inflation measures rather than one is a deliberate choice, and it makes the index structurally slower than headline inflation whenever the three diverge.

Italy indexes to the ISTAT consumer price index for workers' and employees' households, but with two conditions that catch people out. The update is not automatic: it applies only if the contract provides for it and the landlord requests it, and it does not accrue retroactively if forgotten. And the fraction of the index applied is not universally 75 percent — that ceiling is imposed by article 32 of the 1978 rent act on the regulated category and is mandatory there, while for free-market residential leases under the 1998 act the fraction is what the contract says, with three quarters the dominant convention. Landlords who elect the flat-tax regime on rental income give up the right to update for the duration of the election, which is a real economic cost set against a tax saving.

Germany does something different again, and the difference is worth understanding rather than translating. The ordinary route is an increase towards the ortsübliche Vergleichsmiete — the customary local rent for comparable dwellings — justified by a Mietspiegel published by the municipality, by comparable dwellings, or by an expert opinion. Two brakes apply. The rent must have stood unchanged for a minimum period before the increase takes effect, and the Kappungsgrenze limits the total increase to 20 percent over three years, which state governments may reduce to 15 percent in designated areas where the supply of housing on reasonable terms is particularly at risk. The lease may instead agree an index-linked rent tied to the consumer price index, or a stepped rent fixing the amounts in advance; each of those excludes the ordinary route while it runs.

Where the amount is a matter of contract

The United States has no federal rent control, so for most tenancies the answer is whatever the lease says at renewal, subject only to notice requirements that vary by state and to the general prohibition on retaliatory or discriminatory increases. Some states and cities have legislated. California's Tenant Protection Act of 2019 caps annual increases at the lesser of 5 percent plus local inflation or 10 percent, permits at most two increases in any twelve months, and is set to expire at the start of 2030. Oregon's statute caps increases at the lesser of 10 percent or 7 percent plus inflation, permits one increase a year, and the applicable ceiling is published annually by the state — 9.5 percent for 2026.

Notice that even the American caps are structurally unlike the European indices. They are ceilings, not formulas: a landlord may raise by less, and most do. An index tells you the number; a cap tells you the maximum. That difference changes what a tenant should expect from a renewal letter, and it changes what a calculator can honestly tell you. Where the mechanism is a formula, a calculator can produce the answer. Where the mechanism is a cap, it can only produce the boundary, and the actual increase is a negotiation.

Compounding, which is the part that is universal

Whatever your market's mechanism, the increase applies to last year's rent rather than to the original rent, so a tenancy compounds. Take $1,500 a month. Ten years of a 1 percent index gives $1,656.93, a cumulative 10.46 percent. At 2 percent it is $1,828.49, or 21.90 percent. At 3 percent, $2,015.87 and 34.39 percent. At 4 percent, $2,220.37 and 48.02 percent. The permitted percentages sound small one letter at a time; the tenancies they act on are long, and the difference between an index that averaged 1 percent and one that averaged 3 percent over a decade is $358.94 a month, or $4,307.28 a year, on the same flat.

Two national mechanisms are worth converting into the same units so they can be compared. The German three-year cap of 20 percent is an annual equivalent of 6.27 percent compounded, and the reduced 15 percent version is 4.77 percent — both far above any European index, which tells you that in Germany the binding constraint is normally the comparable-rent level and not the cap. Oregon's 9.5 percent for 2026, applied every year for a decade, would take our $1,500 rent to $3,717.34, a cumulative 147.82 percent. And the Portuguese coefficient of 1.0224 for 2026, hypothetically repeated ten times, would give $1,871.97 — a useful reminder that a single year's coefficient tells you nothing about a decade, since it is recomputed annually from fresh price data.

The Italian three-quarters rule shows what a fractional index does over time, and it is not a small effect. If the consumer index rises 2 percent a year, applying 75 percent of it gives 1.5 percent, and after ten years the rent is $1,740.81 instead of the $1,828.49 that full indexation would have produced. At a 3 percent index, 75 percent gives 2.25 percent and ten years produces $1,873.81 against $2,015.87. The fraction is a deliberate transfer: the tenant's rent rises more slowly than prices, so in real terms the rent falls a little every year, and over a long tenancy that accumulates into a substantial subsidy that neither party usually thinks of in those terms.

The gap that opens over a long tenancy

The indexed rent and the market rent are two different series and they only coincide on the day the lease is signed. Suppose the market rises 4 percent a year while the applicable index gives 1 percent. After ten years the market rent for the same flat is $2,220.37 and the sitting tenant pays $1,656.93 — 74.62 percent of market, a gap of $563.43 a month or $6,761.20 a year. At a 2 percent index the tenant is at 82.35 percent of market and the gap is $4,702.50 a year; at 3 percent, 90.79 percent and $2,453.90.

That gap is the single most important number in the relationship and both sides usually misread it. Tenants read it as a windfall and forget that it is exactly what makes moving expensive: the below-market rent is not portable, so the cost of relocating includes the difference between what you pay and what you would pay elsewhere. Landlords read it as a loss and forget that the gap is the mechanism that keeps a good tenant in place, and that a tenancy which never turns over avoids every one of the turnover costs our article on vacancy prices out. A market rent collected for nine months of the year is not obviously better than a below-market rent collected for twelve.

The landlord's decision is not raise it to the cap

Once the permitted increase is known, a second question begins: whether to apply it. Our article on vacancy derives the break-even explicitly — the increase at which the extra rent over a horizon exactly cancels the void and the turnover cost if the increase causes the tenant to leave — and shows that the threshold collapses as the expected horizon lengthens, which is why moderate annual increases usually survive the test and large catch-up increases usually do not. That analysis is not repeated here; what is worth adding is that in an indexed market the landlord's choice is narrower than the break-even suggests, because the size of the increase is not theirs to set. The decision is binary: apply the index or forgo it.

The strategic consequence differs sharply between the two families of system. Where the increase is an index, applying it every year is almost always right: each year's application is small enough to survive the break-even test, and the alternative — skipping years and then facing a gap you cannot close, because in several of these systems the right lapses if not exercised — is strictly worse. Where the increase is a level, as in Germany, the calculation is about how far to move within the permitted band rather than whether to move at all, and the cap defines a speed at which a rent below the comparable level can be brought up rather than a target. And where the amount is free, as in most of the United States, the whole of the break-even analysis applies with full force, because there the landlord really is choosing the number.

Frequently asked questions

My landlord skipped an increase last year. Can they claim two this year?

In several of the indexed systems, no. The right to revise is exercised within a window and lapses if it is not used, and it does not accumulate — France is explicit that a revision not claimed within a year is lost, and the Italian update applies only from the date it is requested. In systems with a level ceiling and a minimum interval, such as Germany, what limits a catch-up is the required interval and the three-year cap rather than a lapse. Where the amount is free, a landlord may propose whatever the notice rules allow at renewal. Check which of those three families your tenancy is in before conceding anything.

Can an index-linked increase be negative?

Arithmetically yes, whenever the index falls year on year, and in a formula-based system the calculation produces a lower rent with no special provision needed. Whether a landlord must apply a fall is a different question from whether they may, and the answer depends on the wording of the clause as well as the statute — some clauses are drafted to operate in one direction only, and whether such a clause is enforceable is a matter for the local law. Deflation is rare enough that the point goes untested for long stretches and then matters a great deal in the year it happens.

Does a renovation change what the landlord may charge?

In several markets yes, through a separate route with its own rules, and it is important not to confuse it with the ordinary indexed increase. Improvement-based increases typically require the works to meet a statutory definition, require notice and documentation, and are computed from the cost of the works rather than from an index — and in some systems they sit outside the ordinary cap while being subject to their own limits. Energy performance adds a further layer in markets that have tied rent increases or lettings to a minimum efficiency rating. If a proposed increase is justified by works, ask which provision it is made under, because it is not the same provision as the annual revision and it is not bounded by the same number.

How do I check the index myself?

Go to the publisher rather than to a summary. The rent reference index is on the national statistics institute's site as a quarterly series; the Spanish reference index is published monthly by the statistics institute to two decimals; the Portuguese coefficient is in a notice in the official gazette and reproduced on the government housing portal; the Italian series is on the statistics institute's site monthly; the German rent index is published by the municipality. Then check three things about the number you have been quoted: that it is the right period, that it is the right series — several institutes publish more than one — and that the arithmetic used is the one your statute prescribes rather than a plain percentage.

Is a below-market rent a reason to stay put?

It is a cost of leaving, which is not quite the same thing. On our figures, a tenant ten years into an indexed tenancy at 1 percent while the market ran at 4 percent is paying $563.43 a month less than a new lease would cost — $6,761.20 a year that a move forfeits, on top of the deposit, the removals and the disruption. That is a real number and it deserves to be in the decision. What it does not do is settle the decision, because a flat that no longer fits, a commute that has changed, or a landlord who does not repair, can easily be worth more than $563.43 a month. Price the gap, then weigh it against everything else rather than letting it decide alone.