An ALT-1 permit, referred to as an Alteration-CO under the DOB NOW filing system, is a NYC Department of Buildings permit required for major construction work that changes a building’s use, occupancy classification, egress configuration, or maximum occupant load. ALT-1 filings result in the issuance of a new or amended Certificate of Occupancy (CO).
ALT-1 permits apply to projects such as converting a commercial building to residential, combining apartments in a way that reduces the unit count on a CO, subdividing units to add dwelling units, adding floors or enlarging a building’s footprint, and converting basement or cellar space to legal residential use. Filings must be prepared by a registered architect (RA) or professional engineer (PE), submitted through DOB NOW, and reviewed by a DOB plan examiner. Approval typically takes three to six months, and total DOB-side costs run $3,000 to $18,000 in filing and permit fees, with architectural and engineering fees of $10,000 to $50,000 or more. As of January 26, 2026, condo and co-op boards must provide formal attestation in DOB NOW before any alteration permit can be filed.
Planning a major conversion, combination, or capital project? MD Squared runs major NYC projects from concept through CO, coordinating architects, engineers, expediters, contractors, and board attestation. Our Capital Project Management service is built for exactly this kind of complex, multi-month build. Talk to our team today and let’s get your project moving the right way from day one.
