To not know if this was a big deal or not

They sound not very bright. It's one thing to gamble a fiver and win 12k and gamble x20 the original stake again. To squander the whole amount is beyond daft. I'd struggle to respect the person who made that choice.

I’d add another note of caution here even if I’m late to the thread. £12k is a lot for most family budgets, but what concerns me more is his reluctance to discuss it.

I’d be really wary of problem gambling here. He probably genuinely thinks he is fine and that you’re making a fuss about nothing. My ex husband kept telling himself that, and only came clean to HIMSELF that he had a problem after losing six figures from his concealed gambling. It went on for years, sometimes with big gaps. It could happen again…

I wonder if your partner would consider accessing support - he can get free counselling from gamcare. Just to chat about what happened with someone trained in the area. (You can do the same - you absolutely qualify as an “affected other”. The gambling industry puts a tiny bit of its cash into supporting such things so claim this bit of the £12k back!)

And do all the banking/debt checks - they’re important for accountability. He’s broken trust here and you need to safeguard yours and the kids’ interests. It’s not easy and I really hope it’s the last of it, but please be careful and sceptical.

I would completely lose all respect I had for my partner and start viewing them as a man child. I have been in a similar situation, my partner at the time was 18 and new to the casino. It just shows an awful lack of judgement that I personally wouldn’t be able to move past

I'd be fuming if my DH did this.

It sounds like they have an addiction issue and realised and stopped

You imply he won via a lottery ticket - did he then spend 12k on lottery tickets? Did he start other types of gambling? Was it throw 12k on one casino number or did it take a longer time gambling in low amounts, which probably involved further minor wins he continued to then bet.
I think you need to look at how he won and lost and understand from that what gambling is to him before you trust there is no issue at all.
I've seen many people win a few quid on scratch cards and 'reinvest' in more in my local shop, so maybe you can look at it as he won 12k on one horse then just chucked it all on the next in a double or quits type thing, which is reckless and impulsive but maybe not as indicative of an ongoing gambling problem more a lack of forward planning for even himself problem.
However, if doing slots, or bets, whilst you can lose lots of money quickly it would take a while, or a period of solely focussing on your bet of choice to get through the 12k - this wouldn't really be disinterested gambling.
To gamble this amount either way takes a certain gambling mindset though I think - so it's not about the money it's about the thrill. Maybe you've never noticed as he keeps it low key somehow and this just gave him room to 'play'.
If he is now really capable of keeping it very low key then great but accept any gambling money he has, from a fiver a year to 12k a week would never really count as him having money to share as it's got to go back into his gambling.
Generally to answer your question, I think it's pretty poor to keep an unexpectedly lucky £12k to yourself whilst you have a house for all the family to finish off but that's with my view that finances should be fairly well shared though I also think you could have room to spend on yourself if you really, really wanted something and this gave you your only ever chance - I would be concerned if they wanted it to gamble rather than buy a man tool or holiday though tbh.

The minute you won £12k then you weren't 'starting from a balance of -£5', you were starting from a balance of +£12 THOUSAND POUNDS.

Which has all been absolutely pissed away.

If they had agreed an amount to gamble with - and stuck to it - essentially to spend on the 'fun' of gambling, would they have chosen £12k to do this with? Or could they have chosen to put aside £500 for gambling with then left the rest of the HUGE amount for other things?

That's definitely gambling problem territory.

They don't change. Gamblers will piss away their life savings and their family's wellbeing before they give a shit. And probably not even then.

Was this some time ago? Do you have concerns now?

You are making a lot of excuses for him. And “he isn’t keen to discuss it”!!! He thinks it’s fine, a non event because the money was “a gift”. He sounds like a classic gambler. I once worked with a lovely guy. Very sweet, but he gambled a lot and in the end his wife and kids left him. He came across so normal, but he ruin d their lives at the time.

This reads 10 times worse now you’ve updated that you have kids. He has a serious problem if he thinks gambling away 12k is acceptable when the house his children live in need work doing on it. I’d say you need to look carefully into your finances OP, see if things add up or if there are hidden debts.

They never make it back. And I think you're being very overly generous to him. He was irresponsible and selfish.
He didn't give a shit about your debts or outgoings when he did this and still doesn't if there is still work to be done.

You can’t really say someone doesn’t have an ongoing gambling issue if they gambled away 12 grand.

In answer to your question, I suppose it was theirs to do what they like with but the fact their priority was gambling it away would mean the end of the relationship for me. Sorry OP.

Edited to add, I would be highly suspicious that this wasn’t the first incidence of loosing a large amount of money. I’d be inclined to guess that there are other financial issues that don’t add up.

That’s a lot of money to gamble away, I would be very concerned they would do it again.

I’d be furious that they spent the whole amount on something so stupid. Especially if it was needed elsewhere. I couldn’t be with someone like that. And I don’t believe they wouldn’t do it again if they came into more money.

This would make me question the whole relationship.

Not from a place of greed but just wondering why they wouldn’t want to make your life as a family better.

That 12k could have been a house deposit, an extension, a once in a life time holiday, a wedding, a new car etc etc.

I know that if I won that money my first thought would be “How can I use this to make OUR life better?”

It just shows pure selfish, reckless and down right stupid behaviour.

Of course it's a big deal, because it's an indication that he's a fucking idiot. He pissed twelve grand down the drain. It's not even about what else he could have done with the money; it's the fact that he's a massive fool.

I would be pissed off if it meant they or you had money problems at the time or are short now.

They should have taken the £12k and invested it.

The greed and gambling is deeply deeply unattractive.

Absolutely not easy come easy go if it's money you really need and can use to better your lifestyle.

I'd be more worried their were hidden debts I didn't know about. Someone who can gamble that much away isn't usually someone who can just stop gambling.

Someone who can blow 12k on gambling, doesn't just 'stop'

Look deeper OP

Well. YANBU to be upset.
But YwouldBU to be angry.

With gambling, if you’re prepared to take the wins, you have to be prepared to accept the losses. You can’t be pleased with him for winning and cross with him for losing.

Depends how emeshed your lives and finances are and, to a degree, how well off you both are.

What did he bet on and how, was it one big thing, did he have a swanky trip to Vegas and splurge, or was it loads of spins on a slot machine/ online bets that got out of control?

I definitely wouldn't commit to any shared finances with someone who could gamble a £12k win without even mentioning it! I'd also be put off someone who thought there was money to be made on gambling tbh.

I think this shows a real unbothered attitude towards the relationship/spouse/children/any responsibilities actually.
I'd expect there to have been a discussion about the money, what we could do, what we could have to look forward to, do household things need replacing or diy jobs need to be done rather than just spunk it all away in a heartbeat.

Also a person who can just blast £12k on gambling probably has a problem.