Dale M's answer contains an inaccuracy for England and Wales. I haven't checked the rules for NI or Scotland but my guess is that the principle is the same (i.e. that a notice with the wrong expiry period will be invalid and needs to be re-served).
- You are not based in Wales (you stated 4 months notice period requirement and in Wales the minimum is currently 6).
- You've been given a Section 21 notice. I haven't considered the case where you've been given a Section 8 notice or a notice under a break clause in the contract (although many of the principles set out in this answer will be the same). A section 21 notice is a "without fault" notice which doesn't require any justification other than meeting the statutory criteria, and is the most common form of notice.
- You have an assured tenancy agreement, as defined in section 1(1) of the Housing Act 1988 (the "HA 1988"). This is almost certainly the case if you have exclusive possession of some part of the property, you use it as your main residence, and your landlord does not live in the building.
Pursuant to sections 5(1) and 5(2) of the HA 1988, your landlord cannot take possession of the property without a court order. In order for the court to be able to give an order for possession, your landlord needs to have given you a notice as follows:
- In the case of a notice given before or on the end of the initial fixed period, a minimum of 4 months notice pursuant to sections 21(1) and 21(2) of the HA 1988.
- In the case of a notice given during a periodic tenancy (which is what you will have if the initial fixed period has expired and a new tenancy agreement has not been agreed), a minimum of 4 months notice with the notice period to end on the last day of a period of the tenancy, pursuant to section 21(4) of the HA 1988. Note that in practice this means the notice needs to be between 4 and 5 months depending on which day of the month it is served.
Note 1: both of the above statutory provisions use the phrase "has given to the tenant" in relation to the notice. A notice given to the agent by the landlord is irrelevant.
Note 2: the version of section 21 linked to above on legislation.gov.uk is out of date and does not reflect the 4 month period which is a temporary amendment introduced as a coronavirus measure. You can see the current version if you use a paid service such as Westlaw, or if you consult the Coronavirus statutes which made the amendment.
Any notice which has not been given in accordance with the statutory requirements is invalid and will be rejected by the court. Therefore it is not correct to say that you can stay another 2.5 months.
Rather, you can stay for as long as you like until the landlord serves a valid notice and then you can stay for the duration of the new notice period. Even after the notice period expires, you can stay until the landlord obtains a court order and instructs a bailiff (but if the landlord does either of those things then you can expect the landlord to be awarded an order for costs).
Other factors which may render the notice invalid
If your landlord has failed to provide you with any of the following then, pursuant to sections 21A and 21B of the HA 1988 and regulations 2 and 3 of the The Assured Shorthold Tenancy Notices and Prescribed Requirements (England) Regulations 2015, any purported section 21 notice is invalid even if it gives the correct notice period:
- Energy performance certificate
- Annual gas safety certificate.
- Government's "How to rent" guidance booklet.
Additionally, a section 21 notice is invalid if your property is one which requires an HMO licence and your landlord does not have a licence, pursuant to either section 75 or section 98 of the Housing Act 2004 (depending on which type of licence was required).
Question from your comment: "what should I do if when I come back home the key has been changed? Or something similar? Should I report the case to the police?"
If the landlord, his agent, or any other person attempts to obtain possession unlawfully (i.e. without a court order) then they will commit a criminal offence under section 1(2) of the Protection from Eviction Act 1977.
You may want to consider contacting any of the following: the police, the housing department of your local council, Shelter, Citizens Advice. If you want to be more proactive, consider hiring an emergency locksmith to change the lock again and bill the cost to the landlord.
With that said, prevention is better than cure. If you get the feeling your landlord may attempt something illegal, I would recommend a strongly worded letter sent by email and registered post to your landlord and his agent, notifying them that their notice is invalid and that any unlawful attempt at eviction is a criminal offence. You can cite the statutory references above. You may also want to inform the letting agent that if they attempt unlawful eviction you will report them to their letting agent redress scheme. Hopefully this will be sufficient to discourage them from doing anything other than re-serving a valid notice.