Methodology May 16, 2026

How we built the TCB framework for 421-a phase-out modeling.

When a 421-a tax abatement expires, the building's property-tax bill reverts to full Class 2 multifamily rate. For affordable-tier unit owners who bought at prices anchored to the during-abatement carrying cost, that creates a permanent, structural carrying-cost cliff. The Tier Carrying Burden (TCB) framework is how we measure it.

The 421-a program is a NYC property-tax incentive that exempts new multifamily construction from property tax in declining percentages over 10, 15, 20, 25, or 35 years. When the abatement expires, the building's tax bill snaps back to the full Class 2 (multifamily) rate. The typical per-unit increase is $10,000 to $25,000 per year. For market-rate owners, this is unpleasant but absorbable. For low-income tier owners in mixed-income 421-a buildings, this is a structural carrying-cost crisis. The TCB framework lets us quantify it.

The formula.

TCB = (annual common charges + pro-rata Class 2 property tax + amortized special-assessment reserve) ÷ tier-median household income.

Each component is sourced from a known public dataset. Common charges from the building's offering plan or annual disclosure. Class 2 property tax from NYC Department of Finance (DOF), with pre-abatement, during-abatement, and post-abatement values. Amortized special-assessment reserve from the building's 10-year capital-project history (a proxy for what owners can expect on a forward-looking basis). Tier-median household income from the American Community Survey (ACS) at the income tier the unit was sold at.

The threshold bands.

Federal housing-cost-burden definitions give us four bands:

< 30%: healthy. Owner has comfortable margin for non-housing essentials.
30-40%: cost-burdened. Owner is sacrificing other categories (savings, healthcare, food quality) to pay housing.
40-50%: severely burdened. Routine financial shock (lost job, medical event) likely triggers default.
> 50%: housing-cost crisis. Federal threshold for emergency housing intervention.

The condo-tier-fragility lesson.

Condo-tier mixed-income is structurally more fragile than rental-tier. The reason is simple: rental tenants can be churned at abatement end (lease ends, market rate adjusts, new tenant moves in). Condo owners cannot easily be displaced and bear the cliff personally. The result, in TCB terms: the same building can move from "healthy" to "crisis" for the low-income tier between Year 14 of the abatement and Year 16 of the abatement, without any change in market conditions, owner income, or building operations. The change is entirely driven by the abatement schedule.

When you run the TCB model across a typical 15-year 421-a building, three outcomes typically follow at phase-out:

De facto deconversion. Affordable-tier owners can't carry the post-abatement burden. They sell to market-rate buyers who can. The building stops being mixed-income in fact (whatever the offering-plan covenants say).

Building-level destabilization. Concentrated defaults force the board to absorb common-charge losses. The board responds by imposing assessments. The assessments fall on remaining owners, including other affordable-tier units, accelerating the cycle.

Permanent ongoing subsidy. If the building's bylaws or financing structure prevent open-market resale, the board ends up funding ongoing carrying-cost subsidies for affordable tier units out of common charges paid by all unit owners. A 15-year incentive becomes a 50-year subsidy in disguise.

The Langston case.

Our worked example uses The Langston Condominium (BBL 1020457501), the founding case study of this site and the subject of Perez v. Board of Managers of the Langston Condominium (Index 654287/2025). Verified TCB result for the low-income tier (60% AMI): 28% during abatement, 59% post-abatement. The tier crossed the federal crisis threshold (>50%) in 2023. The detail lives in our case study and the framework methodology is at 10_Research/mixed_income_carrying_cost_framework_2026-05-14.md.

Try the calculator.

The framework powers an interactive calculator at /421-a/. Adjust the inputs (purchase price, abatement vintage, AMI tier, building unit count) and the calculator returns the TCB trajectory year-by-year, with the threshold bands shown as horizontal lines on the timeline chart. The default inputs are generic (a $30M assessed value, 200-unit building, $80K income, $1,000 monthly common charges) so you can see the shape of the cliff before substituting your own building's numbers.

The calculator is free. The per-building TCB modeling for any specific NYC condo or co-op is included in our Level 3 Forensic Audit deliverable at launch.

The policy ask.

The TCB framework supports a policy recommendation we've codified as a draft bill: the Mixed-Income 421-a Affordability Disclosure Act. The draft is at 06_Legislation/mixed-income-421a-affordability-disclosure-act.md. The core requirement: every offering plan filed at the AG REFB for a 421-a building must include a TCB projection table at sale, and the same table must be re-disclosed at every subsequent resale. Buyers in the affordable-tier units would be put on contemporaneous notice of the post-abatement cliff before signing. Currently, they aren't.

The bill also calls for a building-funded affordability-extension reserve for existing 421-a buildings so that the concentrated default cascade doesn't destabilize the whole building when the cliff hits. And it calls for the 421-a program itself to be wound down (the June 2022 sunset for 421-a(16) new permits stands; the recommendation is not to revive under another name). Replacement: direct affordable-housing capital subsidies benchmarked against IBO cost-effectiveness analysis, with mandatory IBO pre-renewal review for any future tax-incentive housing program.