For what I understand he did not get an operating permit from the authorities
"Missing operating permit
The background to the case is that the Crown Prince has adopted and rented dwellings in Semsveien 188 A-E without mandatory completion certificate or temporary permit.
When the matter became known in Dagbladet, admitted Crown Prince Haakon that he at that time had not applied for the necessary approvals."
now there has been an audit and Haakon had to remove some gas tanks and install smoke alarms
Asker kommune på tilsyn på Skaugum - Kronprins Haakon måtte fjerne bensinkanne fra kjelleren under leietakernes boliger - Dagbladet
"After Dagbladet two weeks ago revealed that Crown Prince Haakon has adopted five rental homes on Skagum farm in violation of the Planning and Building Act, went a representative from the municipality of Asker on supervision.
In documents Dagbladet has gained access to it emerged that the municipality during the audit requested that the gasoline that was stored in the basement under Crown Prince Haakon rent homes had to be removed."