An application submitted to the London Borough of Camden by the owners of Utopia Village in Primrose Hill requests transforming 23 of the business units into 53 luxury flats.
The government changed the General Permitted Development Order (GPDO) on 30 May to allow office-to-residential conversions without the need for planning permission.
Under the new system, applicants submit "prior approval" applications, but councils can refuse only on flood risk, contamination and transport and highways grounds and cannot ask for planning gain contributions.
The government had argued that only underused or vacant offices would be affected and the move would revitalise town centres. But campaigners say the Utopia Village workshops, featuring "thriving" small- and medium-sized businesses, provide jobs for up to 250 people, and fear closure could have a devastating economic impact on the area.
The Gloucester Avenue Residents Group has urged opponents to write to planning minister Nick Boles and communities secretary Eric Pickles expressing their concerns.
Labour ward councillor Lazzaro Pietragnoli said that the application has prompted 106 letters of objection, adding: "The government should have exempted occupied offices from the GPDO. The negative impact on the mixed community in Primrose Hill is going to be massive."
Among those objecting, according to the Camden New Journal, is retail guru Mary Portas, who suggested amending the new permitted development rights so they can only apply to offices that have been "actively marketed yet still remain vacant for at least six months".
Office owners Utopia Properties said it is "committed to providing high-quality residential accommodation by converting the existing buildings". Camden Council aims to decide by 10 December.
According to a council report, the borough has now received 59 office-to-residential prior approval applications, of which ten have been refused and 29 granted.
The borough is one of four, along with Islington, Richmond and Lambeth, that has launched a judicial review to forestall the new law.