To feel protective of my house

You need legal advice.
This is your older childs home, you should not be risking this with a new relationship.
A good man will understand this.
I really wouldn't be getting married though if there is any difficulty.
He could buy a small property for himself.

When my children buy houses, this is what I will be strongly encouraging them to do but they’re both starting from Ground zero.
Whereas frankly, he isn’t. He has the added advantage of being able to buy into a property that he wouldn’t have been able to afford as a first time buyer himself. I would imagine.
If that’s not the case, then it’s more palatable

I've read your various posts and your viewpoint is very single minded. There is no hand grenade in my will, our son gets the house and my partner is provided for until he moves on. Hopefully this won't be an issue until our son is grown up and in his own home anyway, but if the worst happens my partner would be living in the house looking after him anyway.
He hasn't put any money into the house and our living arrangement allowed him to pay off substantial debt, in the future he can invest or buy a rental property.

I think you need to talk to a solicitor to see if there is a foolproof way of doing that as a married couple. I am not so sure there is, especially if the marriage lasts more than 5 or so years.

I don't want everything to be mine, I want what's mine to be mine. I was married for over 20 years, I'd be perfectly happy not living with another man again. I don't see if as depriving any man of anything, presumably I'd find a man that also didn't want to live together or get married. Doesn't mean I can't be in a relationship or that I am using anyone.

Hmm, not ideal though is it because you've basically stopped your partner from moving on with his life because you will make him homeless the minute he tries! You've never given him the chance to have a joint home ownership with you, you have called all the shots and told him he's homeless at some point if you go before him - how long is your son going to have to wait for your partner to move out of the house that will be his? You've basically left your son and partner a hand grenade in your will!

Nobody needs to be married, I've been with my partner nearly 23 years without being married but we are joint home owners who both pay towards the house which protects us both!
It's going to be difficult for people of your mindset to have future relationships if you want you everything to be yours and yours alone. As I said on another post I used to know someone who got into a relationship with a divorcee and after a few years she wanted them both to flog their separate houses and get a mortgage together, but he wouldn't do it and it made for a really bad relationship. He'd often slink back to his house and she started to feel like he was using her and her house when he felt like it and not committing or contributing. You can't have it both ways, don't get into relationship if you are going to deprive a partner of everything that's important.

I wouldn't get married, I'm in your situation and won't be getting married, my will leaves my house to my son with a right to live there for my partner until he starts a new relationship. It doesn't mean I love him any less but my house was mine before we met and he benefits from that already.

Protect your assets.

My SIL didn’t. She married the most wonderul, kind man 10 years ago. Only, he wasn’t wonderful and kind for long after they were married.

He’s currently dragging her though court and she’ll probably lose a lot.

Everyone told her not to get married. She owned a house, had two children and was 41. She wouldn’t listen.

The marriage part is irrelevant, I'm not married to my partner but we have a shared house which we jointly own and pay for. The OP needs to decide if she's really wanting this relationship or not because she's gonna break it, it's not going to work in the long term and I've painstakingly explained why such as what happens when the house needs things, things break and need redecorating or if she loses a job - will she still be "it's all mine mine mine" or will she begging him for money! He isn't going to sit around being treated like that forever because it's not normal. She brought a baby into the equation who will not have a home if anything happens to her.

Exactly. After my divorce I have promised myself that I will never get married again. I need to know that my house is mine regardless. It's not only divorce that is a risk, I see so many threads on here where husbands have gambled away fortunes and put the family home at risk, or just fritted away money on useless things. By the time they are discovered so much money has been lost already and it's too late to turn things around.

So it’s about weighing up risk versus benefit
They either stay as they are with zero risk or they potentially improve their situation with enormous risk
Anybody who’s being a single parent previously will tell you which one is worthwhile

But they will suffer because she isn't willing to share a joint, secure home with their father, they won't have more bedroom and garden space to play in because mum won't get a joint mortgage with dad und utilise two incomes. There'll be arguments and resentment because the partner will always feel like a lodger who is excluded from being part of anything, it makes life hell everytime the house needs something or redecorating or new boiler etc. Will he get a say in anything, will he be asked to contribute! It's a long term problematic arrangement. What happens if she suffers redundancy? Will she expect husband to stump up the mortgage payments and bills!

If you get married and then split the assumption will be 50/50 of everything.

Which is why most of us who have been divorced are in no hurry to do it again particularly while the children are under 18

If I make a stupid decision, it’s only me that suffers the consequences. It should not be the OP’s older child or indeed the baby that ends up with an unstable childhood as a result of poor decision-making that we see over and over again on this board.

You talked about yourself and your partner as two separate entities buying properties and that you didn't want to both contribute to the same mortgage. Your comment wasn't overly clear to understand.

Get him to contribute the same as you have and then keep it equal going forward. You can't protect the house if you are marrying him anyway so you should aim to be equal contributors so if it goes wrong it's easy to split again.

And I am not sure why you are bringing any of these issues to me because at no point myself or OP talked about living in two separate households.

A holiday home is one thing but I'm taking about people having two separate houses here in the UK just because one person won't commit. Holiday homes are pretty controversial these days because of the impact to locals.

Why unused?
He had a house we lived in. I had a holiday home abroad which I was renting out. And, as my experience showed, running two separate houses was the right thing to do because, when the relationship ended, there was nothing to divide. We set it up from the start so that we both benefitted from the financial setup despite both being committed to the relationship. Until one of us wasn't... which does happen and not something you can predict no matter what your intentions may be.

This will cause issues down the line because in normal relationships and home ownership, things like renovations, extensions, redecorating or even just buying new three piece suites etc happen, how do you work that when there's a toxic imbalance? Will the OP let him have a say in these things? Will she expect him to financially contribute despite wanting him at arms length of having any ownership!

What if she loses her job! Will she suddenly want him to suddenly stump up for the mortgage payments and bills?

Protect your assets. Positive intentions and trust won’t keep the roof over your head if you get a divorce and he is able to take half of it. You need to put yourself and your children first.

My advice to you would be to not get married.
My advice to him would be to buy his own property.
How you do those two things while living as a family is the challenge you both have to work out. One solution would be to live in his new house and rent yours out. Another would be to live in your house and he buys a rental property.
As you both know, divorces do happen. If you get married and you split up, you could end up with only half the equity of a house and unable to buy another.

There's very few adults that don't want to have any home of their own and live with a toxic partner that forever reminds him that everything is hers! She loves herself more than she loves her kids! They could get joint mortgage and lovely family home but she isn't interested in selling up doing that, so if she dies the baby has a father that's homeless and living one income and her kid will never have the home it could have had. This relationship is doomed anyway!