I too have misgivings about the new Education Centre taking up green space. However, I can report that the Horniman say it is necessary in order to enable children to move between outdoor and indoor environments where they can undertake 'messy' activities involving natural specimens. Apparently this cannot happen in the main museum building because of the need to quarantine anything brought in to avoid risk of infection / damage in the museum's collections. Seems like a reasonable argument and if it enables more use to be made of the gardens for educational purposes then that has to be a good thing.