How would you value / structure this property opportunity?

[another20]

Sister has a 1st floor flat in large purposed built maisonette. She owns the freehold and has PP to convert the roof/loft into a flat. She cannot afford to do the work herself but has offered me the opportunity to either invest in this with her ie she has the plot I have the cash and we just sell on or for me to buy the opportunity outright.

Anyone any idea how we would go about this so it was a win/win for both of us?

As sisters, could you just split the money you make 50:50??

Work out your investment as a percentage once work is done sell whole unit, flat and maisonette then you take your percentages of the profit. Or you buy the flat space of DSis as if it was land with planning permission and she keeps the free hold?

She lives in the maisonette - so wont be moving. So leaves us with two options - I just pay for all the conversion work and they we sell and halve the profits - or I buy the opportunity outright and then pay for the conversion and sell or rent it. If I went the second route how would the cost of the opportunity be calculated?

I'm not an investment surveyor, but I'd expect that the purchase price + conversion costs + other costs = 80% of the sale price.

E.g., if you can sell the finished flat for 200k, conversion costs 60k, other costs 10k, then 80%*200k =160k.
160k-60k-10k= 90k to pay for the space with PP.

Thanks JoJo - thats a really neat way of doing it - hadn't thought of "other costs" - what do you thing that these might be?

Not sure myself... legal fees, estate agents, financing costs etc