Constructing a building without a sanctioned plan is illegal under municipal and town planning laws. It can lead to serious consequences including demolition, fines, and criminal liability.
Authorities like municipal corporations and development bodies can issue stop-work notices, initiate demolition orders, and deny completion/occupancy certificates. The Supreme Court has upheld that unauthorized constructions must be curtailed with iron hands, regardless of investment or time elapsed.

Builders and owners may face:
Demolition of the structure
Heavy fines and penalties
Denial of utility connections (electricity, water)
No registration or resale rights
Criminal prosecution in severe cases

Even long-standing illegal buildings can be demolished if constructed without approval. Regularization is rare and only allowed under exceptional one-time schemes for residential use, subject to environmental and zoning checks