The family of Capt Sir Tom Moore are attending a hearing that will decide if they must demolish a controversial spa built in their garden.
The celebrated fundraiser's daughter Hannah Ingram-Moore and her husband are appealing against a demolition order.
They used the Captain Tom Foundation charity's name on the first plans for the building, with revised plans then turned down.
The Planning Inspectorate is expected to make a decision within six weeks.
Ms Ingram-Moore, her husband and their son Benji sat together behind their four representatives as inspector Diane Fleming, appointed by the secretary of state, started to hear the appeal earlier.
The building on the site of the family home in Marston Moretaine was originally approved for the use of the occupiers and the Captain Tom Foundation and had received planning permission in August 2021.
It had been partly constructed when revised plans were submitted to Central Bedfordshire Council in February 2022, which included a spa pool, toilets and a kitchen "for private use".
The revised plans for what was called the Captain Tom Building were turned down by the council in November 2022.
A demolition order for the now-unauthorised building was issued, the council said.
A small group of neighbours attended the meeting, with one arguing that the building was "49% bigger than what was consented".
He said it was close to his property, and was "very brutal".
In documents submitted for the appeal, the family said the structure was "no more overbearing than the consented scheme".
In a written appeal statement, Mr Ingram-Moore said the heights of the approved and built buildings were "the same".
Ms Fleming said that when the council issued a demolition notice in November 2022, the now-unauthorised C-shaped building was "substantially complete".
The spa facility was constructed on a disused tennis court.
Richard Proctor, planning enforcement team leader for Central Bedfordshire Council, said: "Yes, the tennis court wasn't ideal but it was significantly less harmful than the building."
Chartered surveyor James Paynter, for the family, said the scheme had "evolved" to include the spa pool.
"The spa pool has the opportunity to offer rehabilitation sessions for elderly people in the area," he said.
The inspector indicated she would make a site visit, accompanied by representatives for the appellants and for the council.
A written decision is to be published at a later date, within six weeks of the one-day hearing.
In September 2023 Ms Ingram-Moore said in a statement on Instagram that no charity money had been used to construct the building.
Capt Sir Tom raised £38m for National Health Service charities by walking laps around his Bedfordshire garden during the first Covid-19 pandemic lockdown.
He died in 2021, aged 100, and his family set up a separate charity in his name.
It is currently subject to an inquiry by the Charity Commission amid concerns his family may have profited from using his name.
