Can my Ex stop me building an extension?

[HannaCollider]

My Ex and I agreed when we separated (amicably ...ish) that DD and I would stay in the house we jointly own (no mortgage) until DD turned 18; I'd then buy out his half, one way or another. I have almost negligible income.
DD will be 18 next year and my plan is to get my father to buy out Ex and pay for a ground-floor extension to the house for him to live in where he can maintain independance within my care. The best sequence for both me and my dad would be for us to build the extension first and move my dad in, then sell his unoccupied flat and pay off the Ex. My Ex says he won't allow me to build anything while he is still a co-owner. He wants me to pay him off first. We have no written contract or agreement beyond the title deeds of the house and he wouldn't be paying any of the cost of the extension.
Does he have the right to control what I do to the house?

He still owns half the house, so yes of course he can say he’d rather building work waits until he’s no longer liable for anything.

If you build the extension first your house will be worth more, and you’ll have to pay him more to buy him out.
Madness to want to build before you own!

If you build the extension first, then the house will be worth more money and it will cost you more to buy him out.

Can you buy him out early? Or wait to build the extension? Or your dad stays in the main house with you for a bit?

Cross posted with Jane!

Just bide your time and buy him out next year, then look at moving your dd in. Nothing to say you cannot seek planning permission first, then when your ex is off the deeds get the ball rolling with the extension.

Thank you so much for your replies!
Was hoping to avoid getting the house revalued since I already agreed a figure with Ex, which pays off his mortgage on his new house with GF and is roughly what we predicted our house would end up being worth when we separated 6 years ago. Hadn't considered the increase in value caused by the extension! Ex is more afraid of all the possible risks of him NOT getting his money. He also thinks the car parking side-space on which I'd build is worth more than the extra room of the extension.
My dad is 86 and getting pretty frail so sooner rather than later would be better for him, as he lives alone in a second floor flat on a steep hill. Also house-selling is a pretty lengthy process and dad would be basically camping in my house for that time, since I can't really sell his flat with him in it ,and can't buy out Ex without my dad's flat-sale money.
Can Ex legally stop me extending?

since I can't really sell his flat with him in it

People nearly always sell their houses with someone living in them though? Why is this an issue? He can move into yours the day he completes on the flat sale and then you pay your ex off and get cracking on the extension?

Yes, I think he can. It will need both your permissions.

Mostly everyone sells their flat or house while they're in it.

I don't think you need his permission for planning - anyone can get permission on any property. I recall a lady getting planning permission to demolish the house belonging to the Tesco CEO as Tesco wanted to do a compulsory purchase on her property.

Whether he can stop you building is another issue. Would he even know? Is he close enough to drive past and notice?

Just something else to think about, unless you put your Dad as part owner of the house, if he intends giving the funds to you there might be potential problems should he ever have to go into care?

It might be worth an appointment with a solicitor to make sure everything is arranged to you and your Dad's advantage, but legally correct going forward.

dad sells flat, moves in with you, pay off ex, build extension

dont built before - you will increase the value! plus yes your x is correct

Does your DF need the money from the sale of his flat to buy out your ex? If not, why does he not just buy him out now, you don’t have to wait until your dd is 18. Then build the extension, sell his flat, move in.

I certainly wouldn’t build first as you’ll end up paying him more. Surprised he hasn’t considered that, unless like you say he thinks the lack of driveway space will impact on the price. Maybe suggest it’s revalued now and a price agreed so you can start preparing for the work

As pp said you can draw up plans, get planning permission if you need it, and book a builder. But don’t build on a house you don’t own.

I would 100%.... get your dad to sell. Move in with you. Then Buy him out.
And once you are home... free extend for your house for your dad.

You are inviting trouble by even asking him about this. And the extension will increase the value so you have to pay him more!!!