Three requirements, and an increase that misses any one of them is not enforceable.
Form N1, in writing. The Board's form. An email, a letter of your own drafting, or a conversation does not count.
Ninety days' notice before the increase takes effect.
Twelve months since the last increase, or since the tenancy began.
The timing, worked through
An increase effective 1 January must be served by 3 October at the latest. Count ninety days back from the effective date, and remember that if you mail the notice the Board treats it as delivered five days later. Post it on the deadline and you have missed the deadline.
There is no penalty for giving more than ninety days' notice and every reason to. Four months is a sensible habit, and it gives your tenant time to plan rather than presenting them with a surprise.
How much
For guideline-controlled units, the 2026 cap is 2.1 per cent. For units first occupied after 15 November 2018 there is no cap, though the notice and frequency rules still apply. See Setting the Rent.
An increase served late, on the wrong form, or less than twelve months after the last one is void. The tenant does not owe it.
One thing that cuts the other way: if a tenant pays an invalid increase for twelve months without disputing it, it can become the lawful rent. That is not a strategy to rely on, and deliberately serving defective notices in the hope they go unchallenged is exactly the conduct the doubled fines target.
Above the guideline
An above-guideline increase requires an application to the Board, generally for major capital work, an extraordinary rise in municipal taxes, or new security services. Note that since 1 July 2026 the service deadlines tightened: where you are directed to serve an AGI order, you now have seven days rather than fourteen, and a certificate of service is due within five days of that.
A good tenant paying slightly under market is worth more than a market rent and a turnover. One month of vacancy plus cleaning, painting and advertising usually exceeds a full year of the guideline increase. Take the increase, and take the tenant's view of it seriously.