Opposing the building of a railfreight terminal earmarked for Green Belt land on the outskirts of Radlett could cost as much as £2.5 million.
St Albans District Council, which rejected the application for the terminal in February, estimates its own costs for the forthcoming public inquiry will be between £800,000 and £1m.
It could also have to pay as much as £1.5m for the would-be developer's costs, if the appeal is successful.
Helioslough, the developer of the proposed terminal, has insisted the location it has chosen is the most suitable.
But those protesting against the scheme say the development will have an adverse effect on the area.
The hearing will take between six weeks and three months. St Albans council will have to pay for a venue, possibly a rented office in Elstree.
The earliest date the hearing can take place is November, but it could be delayed until early next year.
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