New conference facilities will be built at Stoneleigh Park despite concerns over "creeping" development.
The National Farmer's Union has been given permission to build a single-storey meeting suite with parking next to its headquarters at the site.
Stoneleigh parish council had objected saying the NFU needed "special reasons" to build on the greenbelt and it was not part of the park's masterplan.
But the document, which covers various aspects of development including the need for a new A46 link road and landscape maintenance, has not been signed, so is not applicable.
Speaking at a meeting of Warwick District Council's planning committee last week, parish councillor Howard Baker said: "The park is not the place for offices.
"Stoneleigh already has a traffic problem and the amount this could generate would be intolerable."
An NFU spokesman said the organisation needed additional meeting space because staff from another office are moving to the headquarters.
Some councillors felt the new building, to be built of red sandstone, glass and red cedar boarding with a flat green roof, should be welcomed, but concerns were raised over the masterplan.
Cllr George Illingworth (Con, Kenilworth Abbey) said: "The whole place is changing from the Royal Show ground it once was and the NFU is one of the positive developments on this site.
"I'm delighted it's so successful it needs to expand, but it needs to be done in a controlled way."
Cllr Roger Copping (Lib Dem, Leamington Manor) added: "This site is growing all topsy – it is bad planning.
"We were told it was going to be used for rural and agricultural purposes but there is a car boot sale and private hospital located there."
Supporting the application, Cllr Felicity Bunker (Con, Kenilworth Park Hill) added: "Why should this building be stopped because of an agreement that refers to the whole site?
"We should be welcoming this opportunity.
"It will bring jobs to the area and is not big enough for large conferences and coachloads of people."
Councillors voted to grant the application by nine votes to one.