Three comparables, three months: evidencing market rent from the RTB Rent Register

The three comparables rule reads like a formality. Three examples, from a public register, attached to a notice. Most landlords assume it is the easy part of the new regime and leave it to the end.

Then they open the Rent Register and discover the tenancies they wanted to cite started 8 months ago, the closest match has a different BER, and the rule has a 3 month expiry date built into it that nobody mentioned.

This is how the comparables requirement actually behaves in practice, why the 3 month window changes the order you have to do things in, and what the BER clause quietly did to the value of an up to date certificate.

What the rule actually says

If you are setting or resetting rent at market rate, the statement attached to your notice has to demonstrate that the rent is not above market. You do that with details of 3 tenancies in comparable properties, drawn from the RTB Rent Register, each identified by its Registered Tenancy number.

Comparable means similar in size, type and character, and the RTB names BER as part of character where the property has one. The tenancies you cite must have begun in the last 3 months.

None of that applies if your rent is capped rather than being set at market. On the capped route you attach a printout from the RTB Rent Calculator instead, plus comparables. Which route you are on is decided by facts about the tenancy, not by preference, and the notice checker settles it in six questions.

The 3 month window, and why it reorders your work

This is the clause that changes the job. A comparable is only valid if the tenancy began within the previous 3 months, so your evidence has a shelf life measured in weeks.

The practical consequence is that comparables have to be the last thing you gather, not the first. Pull them in January for a review you serve in April and they are worthless. If you build a spreadsheet of good comparables for your area, it decays continuously and you will not notice, because nothing marks the entries as expired.

It also means thin markets are harder. In a local electoral area where few tenancies start in any given quarter, finding 3 genuine matches from the last 3 months can take real work, and the honest answer is sometimes that the market rent route is not currently evidenced. That is a reason to check before you plan around it.

The BER clause and what it costs

Character now includes BER, and that is a bigger change than it looks. It means a landlord with an out of date or missing BER cannot cleanly demonstrate comparability, because the register entry the RTB is matching against carries a BER and yours does not.

It also cuts the other way. If your property has been upgraded and the certificate reflects it, you are comparing against genuinely similar homes rather than being averaged down by draughtier ones. An accurate BER became a rent setting asset on 1 March 2026, which is not how most landlords have ever thought about it.

For anyone weighing retrofit work, that is worth adding to the sum alongside the energy saving itself.

What the Rent Register now holds

The register is built from RTB tenancy registrations and updates daily. Since March 2026 registration captures more than it used to, which is precisely what makes the comparables test workable.

Each entry can carry the rent, the tenancy registration number, the commencement date, dwelling type, number of bedrooms and bed spaces, local electoral area, floor area and BER. That is enough detail to argue that two properties are alike, and enough to argue that they are not.

The corollary is that your own registrations feed everyone else’s comparables. Registering carelessly, with a wrong floor area or a missing BER, degrades the evidence base you will later want to rely on. Our RTB registration guide covers what the registration itself now requires.

Doing this across a portfolio

For a single tenancy this is an afternoon. For 40 tenancies with reviews spread through the year it is a recurring research task with a hard deadline attached to each one.

The pattern that works is to treat comparables as part of the notice run rather than a separate exercise. Identify the review date, confirm the pathway, then pull comparables in the same week you serve. Anything else invites using stale evidence, and stale evidence is indistinguishable from good evidence until someone checks the commencement dates.

Letting agents carry this for many landlords at once, which multiplies both the work and the exposure, since the agent is usually the one who assembled the file.

Four ways landlords get it wrong

Using asking prices from listing sites. An advertised rent is not a tenancy. The rule asks for registered tenancies with numbers, and a screenshot of a portal listing does not satisfy it.

Reusing last year’s comparables. The 3 month window kills this, and it is the most common error because the spreadsheet still looks fine.

Matching on rent rather than on property. Comparables prove the property is alike. Picking the 3 highest rents in the area and hoping is exactly the behaviour the rule exists to stop.

Ignoring BER. Where the property has one, it is part of the comparison. Leaving it out weakens the statement even when the other attributes match.

How Rentalize handles this

Rentalize keeps the comparables you used attached to the notice they supported, with their commencement dates, so a file assembled in March cannot be quietly reused in September. The evidence bundle, the notice and the RTB submission confirmation stay together against the tenancy.

The workflow sits in rent review software and is used across Rentalize Core and Rentalize 360, depending on portfolio size.

If you would like to see how the evidence bundle is assembled, you can book a 20 minute walkthrough.

Frequently asked questions

How recent do RTB comparables have to be?

The tenancies you cite must have begun within the last 3 months. Older tenancies do not qualify, which is why comparables cannot be prepared far in advance or reused across a portfolio.

Can I use asking rents from property websites?

No. The rule requires registered tenancies from the RTB Rent Register, each identified by its Registered Tenancy number. An advertised price is not evidence of a tenancy at that rent.

Does BER really matter for comparables?

Yes, where the property has one. The RTB names BER as part of the size, type and character comparison, so a missing or out of date certificate weakens your statement.

What if there are not 3 comparable tenancies in my area?

Then the market rent route is not currently evidenced for that property. Check the position before planning around a market reset, because thin local electoral areas genuinely can fail this test in a given quarter.

Do I need comparables if my rent is capped?

The capped route requires a printout from the RTB Rent Calculator showing the calculation, along with details of 3 comparable properties. The market route requires comparables proving the rent is not above market. Both attach evidence, but the evidence is doing a different job.