To be livid with father in law

As others have kindly pointed out, he did text to ask you if it was OK and that was your opportunity to say no.
I get where you are coming from, sometimes family, in-laws, friends can be very intrusive and oblivious. But on another day, he might text about the grass and you are happy that he’s done it.
He isn’t a mind reader and isn’t to know how you are feeling, just as much as you don’t know why he says or does something. Maybe he does these things because he needs to feel useful and wanted and do something nice for his family.
We are all guilty at times of getting wrapped up in our own feelings and being angry at others because of it. On this occasion I don’t think your FiL did anything wrong, and next time he (is polite enough to ask which is more than my PiL do, they just walk in uninvited and unannounced) texts you and you aren’t in the mood for him to come round, just say so.

It isn't a failing try having some stock phrases like can you leave it till next week so you are not saying no, you might need your husband to say something to his dad, but it sounds like he thought it was fine to come and do it

You should have said no rather than ignore!

That said, as someone who very much needs their own space and privacy I totally understand your annoyance and he shouldn't have just turned up and let himself in.

I'd communicate (in person or via text) that you appreciate his help but please wait for a "yes" next time rather than just turn up.

I think thats a bit much, he's just cutting the grass.

As you say he's a bit of a bored old man looking to feel useful.

If your MIL popped in for a cup tea would you accuse her of controlling your liquid intake?

I think it would have been better to have replied no particularly when you describe the type of person he is. He assumed he was right and therefore came over.

Your or your partner need to sit him down and explain that the key is only to be used in a emergency/at your request and not to let himself in at will. You also need to tell him that he needs to wait for a reply to his messages. Thank him for cutting the grass but tell him that your or your partner will being doing it in future.

She shouldn’t have to respond instantly to any incoming message / it’s rude to expect that

and since she didn’t say yes, he was rude to come and do it anyway

would drive me mad

Although he may just be caring and bored rather than controlling / if you are not use to caring parents it may be hard for you to judge

I don’t understand why they have a key if you don’t want them popping in whenever they want.

I would absolutely hate that situation. So no key.

So you make an excuse - No sorry today won’t work, I’ve got a client meeting / big deadline so just need to take advantage of the quiet, thanks for the offer!
Set some boundaries if it’s going to make you this angry - I know from experience how it feels! So get comfortable saying no!

Bit scary how many people here seem to think absensce of a "no" is consent...

You should have responded asap “no it’s ok thanks”. On the other hand if he wants the job of cutting the grass let him crack on

Take it off them or change the locks is the next stage. Say you lost yours or just be honest and say you don’t other people having a copy of your key to jus let themselves in when they feel like!!

Whats happened with your mum isnt your father in laws fault.

Yeah he probably shouldnt have just turned up, but all could have been very easily avoid with a simple text response.

This. Even when he turned up you could still have said no.

Hmm I don’t know.

On the one hand, it’s a very nice thing he’s doing for you but he shouldn’t have turned up without you agreeing to it.

You also could’ve messaged back saying it wasn’t a good time.

I am a bit torn though as he’s only in the garden.
I think if your MIL had done this though, we’d all be up in arms because they normally overstep.

I think I would let it go. He was trying to be nice

why has he even got a copy of your key?????

I would assume he thought you were out since you didn't reply so just carried on, not realising that 1) You were in. 2) You were in no mood for visitors.
And 3) That you didn't want him to cut the grass.

I have no family on my side either and that would make me livid. Must be a thing. We get so used to autonomy.

I think if someone’s overstepping we see it so clearly, where most people just take it as part of life because they’re used to it.

I think you're being unreasonable. He's obviously perfectly used to coming round and doing jobs for you including cutting the grass. He doesn't have to do that. He was being nice not nasty.

I understand completely.
My own late FIL once let himself in and woke me up. It was 10 am..........I had just come off a run of 6 nightshifts .
We took the keys off him.
He had left a message on the answering machine ( which I had slept through) but because no one got back to him he came anyway. Massive violation.

A horror story in one sentence.

Can you get the key back from them? Say it invalidates your home insurance or something.