Thanks for your question.
In the event that you cannot afford to pay him off for the monies paid for the extension the onus would be on him to establish an interest in the property.
If you are unmarried, his only option would be to make an application to the Court to claim an interest in the property under the Trust of Land and Appointment of Trustees Act. You can do this for the type of money he has spent on the property if it has increased it's value.
However, he would certainly need a solicitor to make the application for him because it's quite a complicated area of law. He would have to pay their fees. If he does nothing then you have absolutely no problem at all because nothing would happen. The onus is on him.
If he is able to do this, and note this would take at least months, he would then have to litigate on how/when he is to receive money. He would have to make an application for an order for sale if there was no other option, but the accommodation of the children would be a very important concern of the Court.
It's not going to happen as quickly as you might think so you should try and relax a bit. Litigation in this area takes a long time.
If you can show that you have been reasonable and can pay a proportion of the monies owed with the balance to be paid at a later date then the chances of the court making an order for sale would be considerably reduced. He may instead get a second charge on the property which you can repay later.
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