Here is what Staten Island and Brooklyn homeowners need to understand before renting out a below grade space - the rules, the risks, and why the right professional guidance matters more than a finished floor.
This is the single most important distinction, and it trips up almost everyone. The city defines the two spaces by how far they sit below the sidewalk. A basement has at least half of its floor to ceiling height above curb level. A cellar has more than half of its height below curb level. The words get used interchangeably in everyday conversation, but the DOB treats them very differently. A cellar is generally not permitted to be used as living or sleeping space at all, which means a great many rented below grade units around Staten Island and Brooklyn simply cannot be legalized in their current form.
Even a true basement has to clear several requirements before it can become a legal apartment. These rules exist to keep tenants safe, and skipping them is exactly what turns a rental into a liability. In broad strokes, a habitable basement dwelling needs adequate ceiling height, a proper way to get out in an emergency, enough natural light and fresh air, and fire safety features that protect the people sleeping there.
I have sat across from sellers who spent real money finishing a basement, rented it for years, and then discovered at the closing table that it was never legal. The exposure is bigger than most owners expect, and it does not go away just because a unit has been quietly rented for a long time.
Legalizing a basement is a professional job, not a weekend project. In practice you hire a licensed architect or engineer to survey the space, confirm whether it even qualifies, and prepare drawings. An expediter often helps move the filing through the Department of Buildings. Permits are pulled, the work is inspected, and only then can the certificate of occupancy be amended to recognize the unit. Every step is designed to prove the apartment is safe and legal.
The policy landscape is shifting, too. Past basement conversion pilot programs and the newer City of Yes for Housing Opportunity framework aim to make more accessory dwelling units possible across the five boroughs. That is genuinely encouraging for homeowners, but it is not a blanket permission slip - which neighborhoods qualify, flood zone conditions, and DOB approval still control what is actually allowed at your address. If you are weighing the numbers, my ADU income calculator and two to four family house hack calculator can help you think it through, and the resources page collects more local tools.
A quick but important note: this page is general information, not legal, tax, or financial advice. Every property is different, and the only reliable answers come from the DOB and from licensed professionals - an architect or engineer, an expediter, and where money or liability is on the line, an attorney. Please confirm your specific requirements with them before spending a dollar or signing a lease. If you want a grounded, no pressure conversation about how any of this affects your home's value or your plans, reach out to me directly.
Not automatically, but it is illegal to use as living space unless it is a legal basement (not a cellar) and the certificate of occupancy specifically allows a dwelling unit there. Many rented basements and cellars in Staten Island and Brooklyn were never approved by the DOB and do not appear on the C of O, which makes them illegal to occupy or rent even if they look finished.
The difference is how far the space sits below the sidewalk. A basement has at least half of its height above curb level, while a cellar has more than half of its height below curb level. Cellars are generally not permitted to be used as living or sleeping space, which is why so many below grade rentals cannot be legalized as they are.
There is no single number. Costs vary widely depending on the architect or expediter fees, DOB filing fees, and the construction needed to meet ceiling height, egress, light, air, and fire safety rules. Some homes need only modest work while others need excavation or a second exit, which can push the budget into the tens of thousands of dollars. Get written estimates from licensed professionals before you count on any figure.
City of Yes for Housing Opportunity created a citywide framework for accessory dwelling units, but basement and cellar conversions still depend on which neighborhoods are enabled, local flood and safety conditions, and DOB approval. It is a promising path, not an automatic green light. Confirm your specific address and situation with the DOB and a licensed architect before making plans.
Whether you are planning to rent, planning to sell, or just want a straight answer about the space downstairs, I can help you weigh it before you spend.