REGULATORY GAPS
The 421-a Cliff
Why your tax bill doubles, then triples
421-a delivered affordability through a 15-, 20-, 25-, or 35-year tax break, not through the unit's purchase price. When the break ends, the building's carrying cost reverts to full Class 2 tax. Your income did not. The math was always going to break.
The theory in five paragraphs
The deal on opening day. A developer builds a new condo or rental in NYC. The city offers a deep property-tax abatement — typically 100% exemption for 11-25 years, then a phase-out — in exchange for the developer setting aside a portion of units as "affordable" (rent-capped or sale-priced for buyers below specific income thresholds). The developer captures the construction-period benefit. The first generation of owners or tenants gets housing at affordable terms. The City reports an affordable-units-delivered count. The deal looks like a win for everyone.
The shape of the abatement. 421-a abatements aren't uniform. The oldest vintage (421-a(1) through 421-a(15)) ran 10 to 25 years depending on geography and option. The 2017 reform — "Affordable New York Housing Program," codified as 421-a(16) — runs 35 years on rental buildings. Every variant ends. Every variant phases out before it ends. The phase-out is rarely highlighted at sale: it appears as a footnote in offering plans and gets translated by brokers into "low taxes for X years" — technically true but durably misleading.
The carrying cost during the abatement. While the abatement is active, the building's tax bill is suppressed. Common charges remain real (operations, staff, insurance, elevator maintenance, façade compliance under Local Law 11). But the building's largest single line — property tax — is shielded. Owners across all income tiers experience roughly proportional carrying costs. A market-rate owner at 7% of income, a mixed-income owner at 18%, a low-income owner at 28%. The building looks like a healthy mixed-income community. It is, for the moment.
The phase-out years. After the 11 years at full exemption on a typical 15-year vintage (DOF code 5113), the exemption steps down 20 points per year — so the taxable share rises 20%, 40%, 60%, 80% across the next four years, and the unit becomes fully taxable the year after the term. A building whose total assessed value is $22M (the Langston, BBL 1020457501) goes from $0 in property tax through the full-exemption years to roughly $540K, $1.1M, $1.6M, $2.2M across the four step-down years, then full tax the year after. Divided across 180 owners, the per-unit annual tax climbs from $0 to $12,300+ over those years. For an owner earning $50,000, that's a housing-cost increase arriving from a line item they didn't budget for and weren't warned about.
The cliff. The year after the term, the building pays full Class 2 multifamily tax in perpetuity. The market-rate owner absorbs it (their carrying cost stays under 15% of income). The mixed-income owner feels it (around 37%). The low-income owner cannot absorb it — their carrying cost hits 59% or higher, well past the federal 30% housing-cost-burden threshold and into the range where default risk is material. Three outcomes follow: (a) De facto deconversion — the low-income owner sells, but the buyer who can afford the post-abatement cost is a market-rate buyer, so the unit transitions out of mixed-income; (b) Building destabilization — the owner defaults, the building absorbs the lien write-off, and special assessments shift onto remaining owners; or (c) Permanent ongoing subsidy — the City extends the abatement, in which case the "15-year incentive" was always a 50-year subsidy. There is no fourth outcome.
Cross-references: Class 1 vs Class 2 disparity is the underlying tax-architecture problem this builds on. The Langston (Case Study 001) and 15 Hudson Yards (Case Study 002) demonstrate the cliff at different income tiers.
The typical 421-a arc, by vintage
Different 421-a vintages have different phase-out schedules. The calculator below models the four most common variants:
| Vintage | Years 100% exempt | Exemption step-down | Fully taxable |
|---|---|---|---|
| 421-a(15) 10-year uncapped (code 5110) | 2 | 80/60/40/20% biennial | year 12 |
| 421-a(15) 15-year uncapped (code 5113) | 11 | 80/60/40/20% yearly | year after term |
| 421-a(15) 20-year (code 5116) | ~21 | 80/60/40/20% biennial | year after term |
| 421-a(15) 25-year (code 5114) | 25 | 80/60/40/20% yearly | year after term |
| 421-a(16) 35-year (Affordable NY, code 5121) | 25 | flat residual = % of affordable units (yrs 26-35) | not during 35-yr term |
Source: NYC Department of Finance, 421-a benefit exemption schedule (DOF codes 5110/5113/5114/5116/5121); NY RPTL § 421-a; NYC Admin Code §§ 11-245.1, 11-245.1-b. Legacy 421-a(15) benefits step the exemption down 20 points per period (taxable share rises 20/40/60/80) and the unit becomes fully taxable the year after the term. 421-a(16) 35-year projects carry a flat residual exemption tied to the affordable-unit percentage through year 35 — wage-requirement projects stay 100% exempt for the full term. Simplified for clarity; consult a property-tax attorney for your building's exact schedule.
TIER CARRYING BURDEN CALCULATOR (v2)
Find your building's breaking point
Enter your building's 421-a details and your own carrying cost. The calculator projects your Tier Carrying Burden (TCB) — annual housing cost divided by household income — across the full abatement and phase-out timeline. Models vintage-specific phase-out, common-charge growth, optional one-time special assessment, and income trajectory. Federal 30% housing-cost-burden threshold marked; above 50% is crisis.
Disclaimer. This calculator runs entirely in your browser. Nothing is transmitted, saved, or seen by anyone. The TCB formula is an analytical model derived from primary-source NYC tax data; it is not legal or financial advice. Default values model a generic mid-density 15-year-421-a building; replace with your own. For specific building research, see our forensic audit reports. For the methodology deep-dive, see the blog post "How we built the TCB framework for 421-a phase-out modeling."
Your building
Your unit and income
Special assessment scenario (optional)
Scrub timeline
Your year-2026 carrying burden
Your TCB across the full timeline
under 30% · healthy 30–50% · burdened over 50% · crisis
Show year-by-year detail (raw numbers)
| Year | CC | Tax | Assess | Income | TCB |
|---|
What this calculator can't model
This is an analytical model. Real building economics are noisier than a formula. Specifically:
- J-51 stacking. If your building also has J-51 (rehabilitation tax abatement), the phase-out schedule and end date may differ. This calculator is 421-a-only.
- HDFC Article XI is different. If your building is an HDFC co-op, income limits and tax structure differ from 421-a — different regime entirely.
- Assessment value changes. NYC reassesses Class 2 buildings annually. Your real future tax base will differ from today's assessed value (usually up).
- Effective tax rate varies. The class share is reapportioned each year by the NYC Council. The 12.2% default reflects recent cycles, not a fixed law.
- Income tier may differ at your building. NYC AMI is family-size-indexed and updates annually. Our auto-classification uses three rough buckets for orientation — your building's actual tier brackets per the offering plan are authoritative.
- Property tax is a deductible expense. If you itemize federal taxes, your effective post-abatement tax is roughly 65-75% of the gross number (depending on bracket + SALT cap). Calculator shows gross.
- Special assessments can be lumpy AND repeated. Calculator models one optional one-time assessment. A real building may face LL11 + LL97 + roof + elevator + façade in overlapping cycles.
- Common-charge growth isn't smooth. Real CC often steps up in a single year by 10-15% after a difficult fiscal year, then plateaus. The 4% default is a long-run average.
- The 421-a phase-out schedules are simplified. Exact percentages for each vintage's phase-out are spelled out in NYC Admin Code § 11-245.1 and HCR regulations. Consult a property-tax attorney for your building's actual schedule.
This calculator is for orientation, not for legal or financial planning. For per-building primary-source research with the actual phase-out values from your DOF tax history, see our Forensic Audit Reports.
The reform that would fix this
A Mixed-Income 421-a Affordability-Disclosure Act, in draft at our legislation page, would require every offering plan filed at the NY AG Real Estate Finance Bureau to include a Tier Carrying Burden table — exactly this calculation — at sale and at every subsequent resale. A low-income buyer would see, before signing, that their year-21 carrying cost is projected to be 59% of their income.
For 421-a buildings already in existence, the proposed reform would require a building-funded affordability-extension reserve so that low-income tier defaults do not destabilize the building when the abatement ends.
This sits alongside two other CCNYC legislative drafts: a Resident Disclosure Act and a Sponsor-Unit COPA. All three address gaps in the same statutory architecture — RPTL § 421-a, the Martin Act (GBL § 352-e et seq., which governs the new-construction condo offering plans filed with the AG Real Estate Finance Bureau), and NYC Admin Code Title 26.