But he is, as we don't need to sell him.
If your house is worth £500k and you are ok with the mortgage (sure it's a struggle at times, but you are ok with it), then if someone offers you 500k for it, you'll say no thanks, it's ok. If they come back with 750k you may think hmmm, maybe I am open to a move. Not only that, if your house is on a bit of land with approved planning permission then the potential is more. So yep, it is valued at 500k, you don't need to sell, but it has the potential to be worth more to a developer who may say to you I will give you 750k for it. It's that potential Spurs are paying for. Using this same analogy, imagine if you said ok it's yours for 500k and then 2 years later the developer has worked on it and you saw it for sale for £1.5 million.