Over 2,000 titles deeds for plots on riparian reserves will be revoked. Nairobi Governor Evans Kidero gave a directive to the National Land Commission to ensure that the process is done as fast as possible.
Kidero further said that the building opposite T-Mall will be demolished.
Residents and environmentalists have complained that the five-storey commercial building obstructs the flow of the Mutuini-Ngong River causing flooding during the rainy season.
The Governor said the demolition of unsafe buildings will continue after a seven-day grace period to allow those affected to look for alternative accommodation.
Kidero asked developers of unapproved buildings to apply for permits. The structural integrity of the buildings will be assessed before approval.
He spoke during the launch of a Sh4.5 million community cooker at the Kariobangi Social Hall Grounds on Wednesday.
Most buildings, especially those in Eastlands were not approved because the owners lacked title deeds. Many developers bought land from land buying companies that issued them share certificates.
The law previously required one to have title deed to seek building approval.
Owners of the more than 50,000 buildings in Nairobi that were constructed without City Hall approval have three months to seek fresh permits for their property or risk demolition.
The directive came nine days after the collapse of a six-storey building in Huruma estate, killing 51 people ; 140 were rescued from the rubble of the block that crumbled following heavy rains in Nairobi.