Why do people feel ok to extend their stay / outstay their welcome? And how to handle it .

I live in an area people like to visit too. The requests are constants!

You need to ask, upfront, before you agree to the visit, what time they will be arriving and leaving. Most people fly to where I live, so I get them to send me their flight information or train timings at the time of “booking” their visit.

“Oh but we don’t know, there might be traffic” - for those who drive. Well Paul, plan to be here at 2pm and it’s not an issue if you are delayed, I will be in waiting for you. Don’t turn up early, I will not be at home.

“We will leave at 11am” - great, they are free to leave anytime before that, but at 11:15 I have a commitment for which I have to leave too, and my spare keys need to be inside the house when I lock it. So guests leave then.

You can arrive late and leave early, but not the other way around.

This sounds regimented but really isn’t - just enforces boundaries and obviously I remain flexible when it’s a close relative or friend that I genuinely don’t mind hosting for longer than planned. People who don’t respect your time and plans can be quite annoying!

i cant / struggle to find a reason to say no

You don't need to find a reason; simply say "that won't work for us", and if they're rude enough to ask why not tell them you have "some personal stuff going on"

Anything further - *Oooo you won't notice we're here" - just revert to it not working for you

”we agreed by text before hand it was a weekend”

Fine - so you are going to have to be more specific. If it’s too much for them to drive there and back on consecutive days (eye roll) then say, “Come Friday evening so we can do xxxx on Saturday, then I’m going to have to ask you to leave on Sunday because we’re out on Monday and I’m not comfortable leaving you in the house.” As for Christmas, just say, “Please come Christmas Eve until the day after Boxing Day, but we’re visiting friends after that so we won’t be here.” You have to agree an end point in advance, and if they say they “don’t mind” being left home alone or whatever, just say sorry, that doesn’t work for us: I need to be sure the house is locked up or whatever. They are trading on your politeness, so unfortunately, you have to develop a rhino-like hide to deal with them.

I think this is good 👍

This.

Why would you not do this?

If they try to change their minds, say "no that doesn't work for me".

They're being incredibly rude - why are you so worried they might think you are rude? Whether they realise it or not, they're taking advantage of you.

My sister and I get on really well and are quite blunt to each other.

These are not people who are getting hints so I think you have to be just as thick skinned and brazen as they are!

Come for the weekend, but we can only have you stay Saturday night.. 'oh but it's too far to drive, we will need to stay til Monday and have lunch' Sorry that doesn't work for us, we can only do Saturday night.

When they ask to stay for the weekend you just need to tell them they need to go before the working week.
Depending who they are you need to have a conversation with them and say that you enjoy your company but you can’t cope with their visits extending. Monday morning is not the weekend. They had a real cheek asking for lunch on the Monday. They were just trying to extend their day. Tell them to get lunch on the way.

I had a hard time trying to establish boundaries with my dsis who lives overseas. I’d say 3 days, she’d agree then the next person she’d visit would cancel and she’d assume I’d be ok with her staying another week and of course I didn’t feel I could say no. It took 3 years for her to realise and respect my boundary - she thought family didn’t count 🙄

Are these friends or relatives OP? Either way, you're best bet is to lie because they sound thick and selfish as shit. Firstly tell them that they can stay Friday and Saturday night but that Sunday you're holding an Ann Summers party or a seance at your house and every room, including the bedrooms, will be needed. Personally I'd stop inviting them but I realise that's not going to work if they're your in laws.

Tell them you are going away for Christmas but be vague about where. Then don’t actually go away. You can pretend you got ill so couldn’t go away. Nice cosy Christmas at home by yourselves.

I realise this wouldn’t work more than once.

Can other relatives not host them at Christmas ?

If they ask to stay longer you can say

'No I'd like to stick to the plan, it works better for me'

If they push it

'It costs a lot to host so I can't extend your stay'
'I need some time to myself'
'I'm happy for you to come but only if you leave at the planned time'
'I feel put upon'
'I'm happy for you to ask as long as you're happy to respect my answer'

etc.

I learnt to do this with my B and SIL but then they changed tack and said we will drive back overnight when its quiet 🤣
They would sit there yapping until 11.30pm
Oblivious

Have plans made for the day after their stay so extensions are sadly not possible.

You could just say no when they ask if they can stay. Otherwise, give a firm leaving time, 'yes you can stay Friday to Sun morning, I need the room back by lunchtime'. Come lunchtime, strip the bed, move suitcases/belongings into the hall, ask if they'd like a cup of tea before they leave.

Just stop inviting them, say no to a whole year of visits or visit them instead.
I must admit I don't have any problem telling people I want them to leave, I don't mind putting peoples noses out of joint so am similar to youe visitor but the opposite as I rarely if ever impose myself on people.

Who are these people? Do you enjoy having them to stay? Can you stop inviting them or say no when they ask to come? They are just taking the piss by constantly trying to extend the visit, even though it is clear that you don't want them to.

Sorry OP, are you saying "they" to avoid "he/she" or is it two people because people keep thinking it's your Mil and Fil but I was assuming your sister who maybe lives alone and doesn't want to go back to an empty house (still not your problem but maybe sees herself as less of a "guest").

I had to call a visit to a close yesterday by basically starting to do the hoovering around our guest, all other attempts at subtly getting them on their way had failed! Said guest is the type of person to say “I must think about getting going” then stay for at least another 45 minutes.
Overnight guests have a limit agreed before they arrive and barring disaster there is no deviation. In this case “no” is a complete sentence as MNetters are so fond of saying

You need the be direct ( not rude ). As I know you don’t think you’re in the way, but just having you/people in the house is a distraction. You’ll need to leave before I do , I don’t want you/people here when I am out. Don’t apologise for any of your choices, don’t offer excuses that they can dispute. Just smile and say No, you’ll need to leave today/ tomorrow/ when we agreed. That’s all. No discussion or debate. Once it’s done once it will be easier if they push their luck again. If they continue, well they are cheeky fuckers and you might have to stop the visits altogether.

there is no "maybe" about it!

Maybe try solving the problem for them like they try to do for you?
"Can we stay Sunday night it's such a long drive"
"I'm afraid not, we planned it would be just the Saturday night"
"Oh we won't be in the way"
"You'll be ok! You can take it in stages, stop at the service station on the M1..."
"But Bill doesn't like driving at night"
"It's very brightly lit... actually we can help you get ready now if you want to leave in the daylight?"

You have to be actually prepared to say the word "No" as well. You can say it in a soft, kind tone. Like as if it was to a child that you need to hold a nice kind but firm boundary for.

"We can't get ready now, it's much better if we stay till tomorrow"
"Well it would be nice to carry on having fun! I get why you want to stay. But No, we need to stick to the original plan. Every weekend must end, right?!"

first post nails it. If they ask to stay longer say "so sorry not possible"

I promise you that if you do not set out exactly when you expect them to leave at Christmas, they will arrive for Christmas and stay past New Year. You may have to pretend to arrange that you are leaving for an overnight stay for the day you expect them to leave.

Something like, the weekend is perfect as we're going to stay with our friends/travel for work in very far away town on Sunday evening.