Divers also searched a lake close to the luxury home near Castlerock, but it is understood officers are not looking for any bodies.
A police spokesman said: "Searches are being carried out as part of an ongoing police investigation into serious crime.
"These searches using specialist resources are concentrated on land and a lake in the grounds of a private house at Castlerock."
Howell, 50, is currently in jail charged with the murders of his wife Lesley and the policeman constable husband of his then lover 18 years ago.
Their bodies were found in a car filled with exhaust fumes and it was initially thought they died in a suicide pact.
Detectives launched a murder investigation earlier this year after interviewing the dentist.
His co-accused Hazel Stewart, who later remarried, is also charged with the double murder, but has been released on bail.
Her then husband was Constable Trevor Buchanan, who at the time was an RUC scenes of crime officer.
The dentist's American wife and their ten children have fled to try and rebuild their lives back in the United States and the house was put up for sale.
Howell is also being investigated over an alleged catalogue of serious sex crimes on former patients.
Howell was due back in court again on Tuesday for a further remand hearing.