There were breaches of planning regulations on the estate where Dominic Cummings stayed during his lockdown trip to Durham, officials have found.
The PM's adviser stayed with his family in what he said was a "cottage" on his parents' farm in April.
Durham County Council would not provide details of what regulations had been broken, or when they happened.
But it said no action would be taken as they were "historic", having happened outside the time limit for enforcement.
Councils have a maximum of between four and 10 years to take action, depending on the nature of the infringement.
An investigation into whether the property where Mr Cummings and his family stayed was correctly registered for council tax is continuing.
Downing Street has been asked to comment but previously declined.
The council investigated the planning status after receiving a number of complaints following an online article by journalist Alex Tiffin which found no record of a planning application for a cottage on the estate.
City of Durham Labour MP Mary Kelly Foy also raised the issue with the authority.
The only planning applications listed on the council's website for the farm are for a roof over a swimming pool in 2001, and the removal of various trees.
The prime minister's senior adviser said he spent part of the lockdown in a cottage on his parents' estate at North Lodge, on the outskirts of Durham.
He had travelled 260 miles from London with his wife and four-year-old son to get to the family home of his mother and father, Robert and Morag Cummings.
Durham County Council said it had received a number of complaints about whether the property had the correct planning permission, and whether it was correctly registered for council tax.
Stuart Timmiss, the council's head of development and housing, said: "While there have been historic breaches of planning and building control regulations, current legislation places a time limit on any enforcement measures and as a result no further action will be taken."
However, the council has not specified exactly what breaches of planning and building regulations it did find - and it has not been made clear whether any happened during the time Mr Cummings' parents have lived there.
Mr Timmiss said the investigation concluded the main house had not been subdivided, and that the residential use of an outbuilding for family accommodation did not require planning permission.
He added advice had been given to the Cummings family on building control regulations.
Mr Timmiss added: "We have also looked into the complaints raised in respect of non-payment of council tax and will be passing our findings on to the Valuation Office for its consideration and review."
Mr Cummings said he travelled to Durham because of concerns about childcare for his son, after his wife Mary Wakefield had developed potential coronavirus symptoms. They stayed on the estate for more than a fortnight and he said he developed Covid-19 symptoms while he was there.
Durham Police said Mr Cummings may have breached coronavirus regulations by driving to Barnard Castle during his stay, but the force said it would not take retrospective action.
Mr Cummings had said the 60-mile round trip with his wife and son was to test his eyesight to ensure he could make the journey back to London safely, and that he had acted reasonably and legally.