1. Dear Krasimir, it is your decision whether you want to sell the right to build at the first floor level to your 1st floor property owner. In law, the 1st floor property owner has no right to build an extension there as he doesn't own the rear garden. This is because you own the air and space above the rear garden as you are the owner of the rear garden. Your 1st floor property owner needs to buy the right to build an extension at this level from you as you now own the ability to put an extension at this level above the rear garden. So, it is up to you whether and at what price you wish to sell this right to the 1st floor property owner. Alternatively, you can refuse to sell this right to him and you can use it yourself. However, the choice is yours.
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