I'm staggered how many people have strong views on things they blatantly don't understand. Just as examples:
so if you continue you would have spent 9yrs paying half a mortgage, so i think its fair to have a % of the house if you split,33% seems fair as you would have paid roughly a third 9yrs of a 25yr mortgage
Erm, no! What about the equity she put in?
Scenario 1) Say the house cost £500,000. She paid £400,000 from the sale of her old house and carried over a £100,000 mortgage. No way on earth does he deserve 33% of the house!
Scenario 2) I think this post may have been from before OP's update, so you could think of it another way. Say she bought her home for £200,000 years ago before she met OP (we now know this isn't what happened, but just to help people see how overly simplistic their thinking has been). By the time she met OP the house was worth £500,000 and she still had £100,000 of the mortgage left to pay. So he's helped with half of the mortgage for 4 years and anticipates paying for another 5. But he's only paying 50% of 20% of the value of the property, and if she's spent a lot customising the property to their taste without adding much monetary value...
On date X, when you start contributing to the mortgage, the house is worth £500,000.
Your contributions are £500 a month.
At the point at which you split up/she dies, you've paid £12,000 towards the house. At that point the house is worth £600,000.
You get out your £12,000 back, increased by the same percentage as the value of the house, so £14,400.
What about the interest charges OP's partner had to pay during that time? What about renovations she paid for that improved their living standards and made their life better but didn't add much monetary value? In short, we simply don't know enough to make a ruling on what's fair to expect to get back out in the event of a split or bereavement.
OP, I think it's perfectly reasonable for you to be expected to pay a contribution towards all bills, including mortgage. In fact, I think it would be very much in your best interests, if you anticipate this relationship being long term, for you to split all costs 50/50 so that you benefit from the drop in costs when the mortgage is paid off. If you'd moved into a house she was renting you wouldn't quibble about paying half and the cost would probably be much higher with no change of your 'landlord' later marrying you and sharing assets or willing equity to your children. But it doesn't sound like you're both in this for the long haul. It sounds like your partner is ignoring your concerns about longterm finances and you're benefiting from a low cost of living at the moment whilst squirrelling money away for a future life apart.
OP, I don't see how it's fair to build up your own house deposit by paying your partner less. It's not her fault you lost your home in your divorce, particularly since you gave up equity in your marital home to safeguard your pension and I assume you're not offering her a nice big chunk of that?? Can't you agree a compromise where you save slowly (at a rate of what it would cost you to rent a flat minus what you pay towards the mortgage) for the next five years and then ramp up your savings when there's no mortgage left to pay? Then hopefully you can buy a Buy to Let flat to make you feel more secure and hopefully bring in a retirement income for you both. Or if you're preparing for an imminent split, don't do it whilst shortchanging your current partner. Just leave and pay your own living expenses.