Dh has announced his retirement date and I feel irritated by it

There are good reasons why front line people can retire at 55. I can't imagine himsitting at home doing childcare stuff while they're all at 2ndary school. Don't you want him to enjoy his retirement, pursue interests?
Can you ask him to do all shopping and cooking if he doesn't already?

How old are your older dc?

Mine are now 17 and 18 - end of Y12 and 13. Honestly parental availability and headspace is so important in teen/secondary years. It’s a blessing not to have to worry about child care and know they can sort their own food, but I don’t feel like I parent less.

It seems your concern is that the dynamic of you doing bulk of home tasks won’t change. This is really the problem. Looking in as an outsider, it feels like he could be a present and active parent, run the house and still have lots of time left for hobbies. What’s standing in the way of this?

Being a sahm does generally mean being a Mum, so doing all that's involved when little. Your DH was at work so couldn't do it? I worked pt therefore was able to be a sahm for 2 days a week, which I cherished, much better than my working days dropping off at nursery/breakfast club then after school club.

I have been in a very similar situation, although I was never a sahm - but did (and do) work 4days/week while DH did a Big Man job.
It has actually worked out much better than I expected so sharing a couple of things that helped for us:

1. Gave him a full month before I even started mentioning picking up the jobs and responsibilities that I’ve always done.
2. was very gentle in steering him towards doing stuff and being very helpful in explaining the most efficient way of doing things. Patience when he was a bit shit to start with. I thought of it as a job handover - I’d been doing all this for years so it’s unfair to expect him to instantly think of and know how to deal with the mainly unseen minutiae
3. Trained the children to go to him first for boring stuff - but be wary of making yourself unnecessary. Sometimes the lifts or sporting events etc are where you have the nicest conversations with teens
4. Be genuinely happy for him getting to relax and do some hobbies. And be interested in what he’s up to. Fake it til you make it if necessary!

I will get jumped on by all those who think I shouldn’t have to tell him what needs doing and how. I get that, I really do. But working together, and showing kindness has worked a damn sight better than getting snitty about the cleaning would have done!

Isn't that actually a good thing? He will be available to help more. Do you feel the timing is bad because he should've done it before or later?

He's decided it's the right time, assessed finances etc I would assume, so I don't understand, sorry.

This. You are an adult and if you want to let him walk all over you then crack on.

This response of not willing to talk to your husband proves why you're in the situation you are.

Weak attitude. Leave him or resign to the fact you're not willing to make changes

And why are you allowing him to live off you because he hasn't planned?

Hopefully he’ll surprise you. My dad was an older parent and retired when I was in early secondary. He went from doing not a lot to all cooking, ironing, shopping, washing, sick days, lifts etc. Hopefully you’ll find the same!

Why though? Surely he is a grown up and can deal with it? Dont martyr yourself.

Sorry but OP said she worked once they were at primary school, so I'm not sure your comments are that relevant regarding all the time you have when they're away etc. You also know that the phrase "swanning around" is derogatory and dismissive of what the OP was doing.

I am ex fire service. Is he operational or non-uniformed?
Operational firefighters usually retire at 60 unless for medical reasons. Non uniformed staff can retire at 55 but their pension benefits will take a hit of around 30% reduction (unless in the case of redundancy).
Not sure about police though.
You and he may have crunched the figures, but just thought I’d point this out.

As a teacher, I’m fully aware of how much work it takes to look after a family whilst not working, because in effect I didn’t work in the school holidays. I also did all the ferrying around of the DC when they were children purely because we only had 1 car and DH had to get the bus to work - he was out of the house for 12 hours a day in a physical job. I’m now almost fully retired and although my Dc are adults, I also look after the toddler grandchildren 2 days a week.
So yes, I’d say it’s far far easier being at home with children than going out to work. Some days, we would have a lazy day, some days not. I was able to do all the admin required - DH would take the kids swimming at the weekend and I would use that time for those jobs. Being responsible to no one, having 5 hours free every day when the children are in school 5 x a week, 39 weeks a year gives SAHPs a lot of free time when they can, indeed, ‘swan around’.
It’s laughable that some people honestly are claiming that they have no time for themselves because of all the admin they have to do.

Are you frightened of him op? I don't understand why you are laughing at the suggestion that you should talk to him about his parental role after retirement... this is a perfectly normal conversation to have with a partner!

What I don’t understand OP is that you said you were looking forward to a bit of freedom when your youngest goes to secondary school, but that your DH’s retirement plans mean you’ll be stuck with all that goes along parenting a teen. If you are the one that does all that anyway, how does your DH’s retirement change that?

Nonetheless, in your shoes I’d be having a conversation around the fact that he’s retiring from work, not family life. When you weren’t working, you picked up X,Y,Z, now that he’s not going to be working it’s his turn to do those things.

The issue isnt the retirement or the division of parenting duties or finances over the years. Quite clearly fundamentally you dont act as a team so your issue is a relationship one, this latest issue has just made you see it!

I don’t really understand your logic, OP, as surely he wouldn’t have been able to afford to retire or do anything differently if you were a SAHM? Or, if he carried on working, he would retire after even more expectation on you to deal with your children? When were you expecting him to retire - had you had any sort of discussion prior to his announcement?

Design a spreadsheet with everything that needs doing and allocate 75% of all jobs to him

I’d be interested to know how old you are now and how old you were when you decided to become a SAHP.

That’s 3 things!

It will make your life easier surely, he will do shopping, housework, laundry, look after sick kids ...

The teenage years are different to the primary school years.

The hardest was the studying and revision. Two of the boys were not self motivated. Their father used to shout a lot, not helpful. I kept the fridge and cupboards stocked, confiscated phones and Xbox controller a few times. Weirdly they both worked their socks off at university and have done very well.

So I kept a close eye on studying. Otherwise my DH learnt to cook much better than I did. He watched a lot of James Martin and other chefs/cooks on TV. Googled and printed off recipes. Did the shopping. He still prefers to do the shopping rather than me doing it.

taking care of pre-schoolers full time is hardly swanning around

Swan around for years? You're pulling our legs now aren't you 🤣
OP said she was SAHM until the youngest started school, they're now about to be at secondary.
Do you not value the role of someone caring for babies/toddlers or working while managing all primary age child related stuff at all if you can describe it as "swanning around"?

Agree with others, get it clear that he is handling secondary school and all the teen years stuff. This will include ensuring homework is done, helping to find work experience placements (not as easy as used to be) GCSE choices, school uniform all the drama about PE kit, parents evenings, post 16 choices, social activities support such as transport etc you are now concentrating on your career so great, he will be retired so he will be first point of contact. Make it work for all of you