Breaking Down the Costs - Basement, Garage, and Rooftop ADUs in New York
You’ve decided to build an ADU in NYC - congratulations! Now it’s time to break down the costs.
Let’s dive into the true expenses associated with constructing basement, garage, and rooftop ADUs in New York City.
Basement ADUs
The average cost of building a basement ADU in New York City is approximately $150,000 for 800 sq ft, while a detached backyard unit typically costs $250,000 - .
Why the difference? Even though a basement already has existing walls, it requires extensive upgrades - including moisture control, emergency egress, ceiling height compliance, and underground plumbing and electrical retrofits.
Additionally, NYC’s ADU regulations require automatic sprinkler systems for cellar units and specific window well dimensions to meet light and safety standards.
If your basement is below grade or located in a flood zone, you may not be eligible to convert it at all - basement and cellar ADUs are restricted in high-risk flood areas. That said, if your space qualifies, a basement ADU can still be a cost-effective option. You’re working within your home’s existing footprint, avoiding major excavation, roofing, and foundation costs.
In general, plan to spend $150–$250 per square foot, plus $20,000 - $30,000 for permits, engineering, and design.
Garage Conversions
In NYC, garage-to-ADU conversions can range from $75,000 for simple models to $300,000 or more for high-end or multi-story designs - with most projects falling between $80,000 and $150,000.
This wide range depends on factors such as size, complexity, insulation quality, foundation upgrades, distance to plumbing and electrical connections, and whether you’re adding a floor.
However, not every garage is eligible for conversion. NYC requires that all ADUs meet habitable space standards - including minimum ceiling heights, natural light and ventilation, proper egress, and fire separation from the main home.
Permitting costs typically range from $2,500 to $7,000, and if your garage isn’t already connected to sewer or gas lines, those tie-ins can add another $15,000 or more.
Still, compared to new construction, a garage conversion is often the most cost-effective way to add livable space - especially if you’re aiming for a studio or one-bedroom rental unit.
Rooftop ADUs
Rooftop ADUs in NYC are rare - they’re both complex and expensive to build.
You aren’t just constructing a new unit - you’re also reinforcing the building’s structure to support the added weight, installing a new staircase for egress, adding fire suppression systems, and complying with strict zoning regulations on height and setbacks.
Because rooftop ADUs are still relatively uncommon, there isn’t consistent data on average costs. However, to put things in perspective, a basic roof deck in NYC typically costs $25,000–$75,000.
Now imagine enclosing it, insulating it, adding plumbing, and making it livable year-round - a conservative estimate would be $300–$600 per square foot. For a small 600–800 sq. ft. rooftop ADU, that’s roughly $250,000–$500,000+.
Takeaway
Basement ADU: Ideal for single-family houses that already have below-grade space and meet compliance requirements. Budget: $150K–$225K.
Garage ADU: Great option if you have a standalone garage and don’t need parking. Budget: $80K–$180K.
Rooftop ADU: Possible but highly complex, expect costs upward of $250K–$500K+.
Choose wisely. Build smart. Your ADU might just be the best upgrade your home, and your wallet, ever had.