Detailed plans for a proposed migrant camp are to be drawn up after the Home Office awarded a £3.4m contract.
Construction giant Kier has been hired to design the camp at RAF Barnham, a former nuclear weapons store, earmarked for asylum seekers. The base is in Suffolk but close to Thetford in Norfolk.
The revelation has raised fresh concerns about the amount of public money being spent on the project before a final decision is made.
The Home Office insists no decision has yet been taken.
Up to 931 asylum seekers could be housed at the site.
It is among three former military bases being proposed for asylum seekers as the government vows to move away from hotel-based accommodation.
It has said it intends to use the Urgent Crown Development process, a fast-track planning route which bypasses the local council.
The base, which has most recently been used for police training, is thought to need extensive work, including major upgrades to drainage and electricity.
Kier's contract allows ministers to scrap the work "at any time and for any reason", the Local Democracy Reporting Service said.
There has been widespread opposition to the proposal to use the site, which has included protests outside the base.
Scott Hussey, a Reform UK cabinet member at Norfolk County Council, called on the Home Office to reveal the full cost expected at the site.
''Successive governments have never found enough money to give our own Armed Forces decent accommodation – including when this site was used by the RAF," he said.
"Yet for illegal immigrant accommodation the money flows freely. We deserve to know the full cost before any decision is made.''
A Home Office spokesperson said no money "had been paid upfront for these contracts".
"They simply allow work to be commissioned and paid for only if needed while we assess whether the sites are suitable," they said.
The government has been closing asylum hotels, they pointed out, which they said were costing taxpayers £9m per day.
"Alternative accommodation, including former military sites, has 24/7 security and CCTV in place to minimise disruption for residents."
Talking about the plans, Helen Beck, chairwoman of the Barnham Residents Association, said: "We're not surprised but we are hugely disappointed because at the moment there is a hurdle of 'can this plan go ahead?' Because will it pass the environmental test?"
She continued: "The fact that the government is happy to spend £3.4m drawing up a scheme - that at the moment they don't even know whether it can legally go ahead - is hugely disappointing."
Additional reporting by the Local Democracy Reporting Service.
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