Adding value by changing layout of the flat

I would appreciate advice from someone who follows the strategy of adding value to flats by changing the layout in order to create more rooms. 

I have recently purchased a big one bed flat in a block of flats with the intention to create second bedroom. It is my home, not an investment project.

The layout is great, there is no need to remove or add any walls. All I need is to move kitchen to the living/dining room. I asked the freeholders for permission (to extend the pipes to the new kitchen) and I received the most unexpected answer. If the flat is converted to two bed I will have to pay them a premium because the value would increase. They want a share of the profit!!

I am not selling the flat so I will not realise the profit but still they want me to pay them.

Has anyone been in same situation?

Have they got right to demand half of the ‘profit’?

Thank you in advance

Joanna