Arwyn Morgan was contracted by Enzo’s Homes Ltd to carry out felling next to woodland in Penllergaer last year.
He pleaded guilty at Swansea Magistrates’ Court to contravening a tree preservation order after initially denying the charge.
Company director Fiorenzo Sauro pleaded not guilty and his trial continues.
The court heard a Tree Preservation Order had been put in place to protect the redwood tree at the site in Swansea.
The city council’s tree officer Alan Webster said he had visited the site in November after a local councillor contacted him to say the rare redwood had been felled the day before.
He told the court the cost of replacing the tree would be around £250,000 – but added it was practically “irreplaceable”.
“This is ancient woodland dating back hundreds of years. It is important,” he said.
“You couldn’t replant a tree of this size but this [it’s size] is used to extrapolate a valuation.”
Counsel for the prosecution Annabel Graham Paul said Mr Sauro either knew the trees were being cut down by Morgan or had been “reckless”.
“Mr Sauro is an active and hands-on director with day-to-day involvement in business, and is at least neglectful of what was going on onsite,” added Ms Paul.
Mr Sauro’s denies the charge and his trial continues.
Morgan will be sentenced on 24 September.