Files were found after the attacks on a computer dumped in a rubbish bin, a well-placed source confirmed.
The computer also included a final message from one of the airport bombers, Ibrahim el-Bakraoui.
Belgian authorities have identified all 32 victims who died on 22 March.
Seventeen were Belgian, and the other 15 from around the world. Another 94 people are still in hospital, and dozens of them are in intensive care.
The first funeral has taken place, for Raghavendran Ganeshan, whose body was returned to the Indian city of Chennai on Tuesday.
The discovery of the computer emerged the day after the Brussels attacks. It had been abandoned in a bin in Max Roos street in the Brussels area of Schaerbeek.
The three suspects captured on airport CCTV had left a house on the same street by taxi and the driver had later guided police to the address, where explosive materials were found.
Quoting reliable sources, De Tijd newspaper said earlier that files on the computer contained details and photos of the prime minister's building and office at 16 Rue de la Loi (Wetstraat in Dutch) as well as details of his official residence in Lambermont street.
It is unclear if these were downloaded from the internet or taken by the bombers themselves.
A spokesman for Mr Michel said security had been strengthened well before the attacks last week because of concerns the buildings were vulnerable.
The chancellery is also next door to the US embassy and on the same street as Maelbeek metro station, where Ibrahim el-Bakraoui's brother blew himself up little over an hour after the airport attack.
The computer has become a vital source for Belgian investigators searching for the third airport suspect, who is thought to have fled the airport when his suitcase did not explode.
In his final message on the computer Ibrahim el-Bakraoui complained of being hunted and not feeling safe anymore.
Copies of laptop hard drives linked to suspects in the Brussels attacks have been sent to the FBI, according to reports from the US.
Brussels airport remained closed for flights on Wednesday, after a test involving hundreds of staff. The airport operator said no flights were planned until at least Thursday afternoon as the results of the trial were still being assessed.
A temporary check-in area has been installed along with enhanced security measures. But officials say they will only resume flights when they are able to operate at 20% capacity,
Chief executive Arnaud Feist has said it will take months to reopen fully, as the departures hall will have to be rebuilt "from the air conditioning to the check-in desks".
Several airlines have diverted flights to Liege, Antwerp Ostend and Charleroi. However, Brussels Airlines is running only 40% of its flights and says it is losing €5m a day because of the closure of Brussels airport.
In a separate development, an open letter written by police at the airport has been published by Belgian broadcaster VRT, in which they express frustration and say security was not taken seriously enough.
They complain of a shortage of staff and say the airport's "open infrastructure" meant that trains, buses and cars were able to approach and leave the airport without any control.
Jihadist bombers who attacked Brussels airport and metro last week also collected building plans and photos of Prime Minister Charles Michel's office and home, the BBC has learned.