It sounds as though the developers would have a 'second charge' on your property. You buy the house for, say �100.000. You mortgage for �75,000 but you don't pay any rent for the remaining �25,000 and the developers do not own it. Instead, they have a second charge on your property for the �25,000. In simple terms, the developers 'lend' you the �25,000 but you don't have to pay it back in instalments. What happens usually, is that they'll ask you to come up with the 25% in 2 years (either �25,000 or 25%, whichever is the greater), or in 5 years (either �25,000 or 25% whichever is the greater - plus interest). This may seem like a good idea but WILL NOT be if house prices fall. Back in 1999, we purchased a 'shared purchase plan' house for �52,950 (god I wish they were that cheap now) and opted for the 2 year deal. Within 8 months, our house had fallen in price and, in fact, when we were due to pay back the 25 % (approx �13,000), the house was worth �39,000 !!! There's no way we could raise that money but thanks to a few insurance policies (which we would have rather kept going) we managed! Think carefully!