Radon on an FHA multifamily loan

Section 9.6.3 of the Multifamily Accelerated Processing (MAP) Guide, issued by the Department of Housing and Urban Development (HUD), requires a radon report on an application for Federal Housing Administration (FHA) multifamily mortgage insurance unless one of two exceptions applies. A certified radon professional signs the report, and the threshold is 4.0 picocuries per liter (pCi/L) (MAP Guide, 9.6.3.2.A, 9.6.3.2.D.3). This page rests on the 2020 MAP Guide, revision of March 19, 2021. Mortgagee Letter 2026-04, the only later letter or notice whose title touches the chapter as of October 9, 2026, changes no section cited here.

Who the rule binds

The lender submits the environmental report for every project and provides the cost estimate for radon remediation (MAP Guide, 9.2.1.A, 9.6.3.2.K).
The Radon Professional. Testing of an existing property, testing after construction and any mitigation those tests require must be done by a Radon Professional or under one's direct supervision. The guide requires certification from the National Radon Safety Board (NRSB) or from the AARST National Radon Proficiency Program (NRPP), and a state certification or license where the state of the work requires one (MAP Guide, 9.6.3.2.B).

What it asks

The report and its exceptions:

The report. It must give the results of any testing, the sampling strategy as applicable, and the details and timing of any mitigation deemed necessary. A Radon Professional must sign it, certify that it complies with the section and include copies of certifications or licenses (MAP Guide, 9.6.3.2.A.3).
When it is due. With the pre-application or the application, whichever applies. For new construction, and for a substantial rehabilitation or conversion where early testing is not feasible, at the final completion inspection (MAP Guide, 9.6.3.2.A.2).
Exception: a standard's own exemption. The Radon Professional may find testing or mitigation unnecessary under an exemption in the relevant radon standard, the state's or ANSI-AARST's. The justification goes in the environmental report as a signed letter that references the standard, and Housing staff decide whether to grant it (MAP Guide, 9.6.3.2.C.1).
Exception: excluded refinancing. No radon report is required for an application categorically excluded and not subject to the laws and authorities that 24 CFR 50.4 lists (the guide's CENST level), under the exclusion for "Refinancing of HUD-insured mortgages that will not allow new construction or rehabilitation, nor result in any physical impacts or changes except for routine maintenance" (24 CFR 50.19(b)(21)). Testing is encouraged. An applicant who tests must follow the testing and notification protocols and report the results in a radon report, and a result at or above the threshold requires mitigation (MAP Guide, 9.6.3.2.C.2).

Testing:

Protocol. ANSI-AARST MAMF-2017, which the guide says includes testing every ground floor unit (100%) and 10% of upper floor units in each building of the project (MAP Guide, 9.6.3.2.D.1).
The alternative. Instead of a full testing assessment, every ground level unit or room in each building must be mitigated to the applicable mitigation standard (MAP Guide, 9.6.3.2.D.2).
Threshold. The threshold for unacceptability is 4.0 pCi/L, on initial and any confirmatory testing (MAP Guide, 9.6.3.2.D.3).
Occupants must be told of coming tests in the manner of Section II.B and Section III. 2.2.1 of AARST MAMF-2017, and told before and after mitigation (MAP Guide, 9.6.3.2.E).

Mitigation:

When. All new construction must be radon resistant. An existing building must be mitigated where testing shows radon levels that exceed the threshold (MAP Guide, 9.6.3.2.F.1).
Standards. For existing buildings, ANSI-AARST RMS-MF 2018 (multifamily) and ANSI/AARST Standard SGM-SF-2017 (single family). For new construction, ANSI-AARST CC-1000-2018 or ANSI-ASHRAE 189.1-2017, Sections 801.3.4, 1001.3.1.9, and 1001.3.2.1.4.5.d (multifamily) and ANSI-AARST CCAH-2020 (single family). The guide says they include post-mitigation testing (MAP Guide, 9.6.3.2.F.1). Wherever the guide cites a standard, HUD requires the most recent edition in force or the superseding document (MAP Guide, 9.1.1.D).
Deadlines. For new construction and substantial rehabilitation, mitigation and follow-up testing must be complete, and every report with HUD staff, before final endorsement. For a Section 223(f) project, mitigation that is part of the repairs must be finished as quickly as practicable and no later than 12 months after Closing (MAP Guide, 9.6.3.2.G).
Certificate of completion. One from the Radon Professional must be appended to the radon report once testing, mitigation or both are complete (MAP Guide, 9.6.3.2.H).
Operation and maintenance. Any mitigation project must have an operation and maintenance plan under the mitigation standard that applies. The final plan goes to HUD once the system is installed, and a condition of the Firm Commitment shall require the borrower to follow it for as long as the mortgage is insured (MAP Guide, 9.6.3.2.I).
Existing systems. Any mitigation system already installed must be evaluated for proper function, with corrective action by a qualified radon professional where applicable (MAP Guide, 9.6.3.2.J).

By program:

Section 223(f). Every project must be tested, no earlier than 1 year before the application is submitted, or the applicant may go straight to mitigation. Where estimated costs exceed what the program allows, the application cannot be approved, though it may be considered for the substantial rehabilitation program (MAP Guide, 9.6.3.3).
Substantial rehabilitation and conversion. Every project must be tested. Early testing, where feasible, carries the same 1 year limit: below the threshold, no mitigation is required; at or above it, the project design must include mitigation. Where early testing is not feasible, the project goes straight to mitigation. Where mitigation is not built into the design, a result at or above the threshold requires retrofit mitigation (MAP Guide, 9.6.3.4).
New construction. Testing is required after construction and before the final completion inspection. A result above the threshold means activating the mitigation system or retrofitting (MAP Guide, 9.6.3.5.B).

What it means for the Phase I and the consultant

The regulation. HUD's policy sentence, quoted below, names radioactive substances and not radon. The protocol, the threshold and the standards are the guide's.
Outside the Phase I's scope. The standard lists naturally occurring radon among its non-scope considerations. It requires no assessment of them and lets user and environmental professional contract for additional services (E1527-21, 4.4, 13.1.2, 13.1.5.13).
What the guide's Phase I section leaves here. The guide's contamination section, which holds its Phase I requirements, sends radon's specific requirements to section 9.6.3 (MAP Guide, 9.4.A).

What the rule says

It is HUD policy that all property proposed for use in HUD programs be free of hazardous materials, contamination, toxic chemicals and gasses, and radioactive substances, where a hazard could affect the health and safety of occupants or conflict with the intended utilization of the property.

24 CFR 50.3(i)(1), edition revised as of April 1, 2025

This page is general reference for environmental professionals. It does not review any project. The regulation is the authority, and the determinations of HUD or of the entity responsible for the review govern.