Planning Inspector Joanna Gilbert has dismissed the appeal by Redrow Homes to develop 310 houses, a primary school and community facilities on the land at Harts Farm.
The appeal which was heard in blocks across the last couple of months centred on the use of the green belt land and the flood risk.
Like all green belt applications the issue centred how the development would impact on the openness, purpose, character and appearance of the green belt and whether there were any very special circumstances which would outweigh the harm.
The Inspector dismissed most of Redrow's claims that the green belt wouldn't be harmed and that there was no effective flood risk.
Hertsmere planning officers will study the decision in depth and what the implications are for both Harts Farm and the wider green belt in the borough.