Plans to build a new secondary school on Green Belt land in Croxley Green could make the surrounding area "prey" to future development.
Hertfordshire County Council is pursuing its plans to build a secondary school in a Baldwins Lane horse field.
The council plans to buy the land and there is currently an inspectors report being put together to determine the suitability of the site.
Chairman of the Croxley Green Residents Association, Barry Grant, said there are concerns over the location of the proposed site, particularly as it falls within the Green Belt.
Mr Grant said: "If this goes ahead, it will change the balance and make other Green Belt sites around the area prey to development.
"The Green Belt area forms a function, such as preventing accommodations joining together, which will disappear if you get rid of its status."
The county council’s cabinet member of education, Chris Hayward, said the land north-east of Baldwins Lane will best meet the council’s need for a possible new school.
Councillor Hayward added: "We need to plan ahead so we can meet the rising demand for secondary school places in south west Hertfordshire, otherwise we risk a serious shortfall of places in the future."
Councillor Hayward said that in order "to be as prepared as possible" the council plans to buy the land.
He added: "We are working with the landowners to ensure they get a fair price for their land and hope we can reach an agreement."
The site was originally opposed by Three Rivers politicians when it was put forward by county last year.
However, despite suggesting that building a farm in Maple Cross would be more appropriate, the district council’s executive meeting voted to support the Croxley Green site, with seven votes for and one abstention.
Mr Grant said he understands the need for extra school places, but that there must be a more "suitable" location.
He added: "For the people of Croxley, just because a new secondary school might be built, doesn’t guarantee we will get more school places."
Councillor Hayward commented: "We have looked in detail at other sites, but this one will best meet our needs for a possible new school, especially given its proximity to Watford.
"This purchase will ensure that a reserve site would be available in the event that it is required in the future for a school. If a decision is made to build on this site, it will go through the full planning application process, which will include public consultation with local residents.
"Ensuring all Hertfordshire students have access to a good education is extremely important and we must continue to invest in our school infrastructure."