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Unit-count math

How many apartments need radon testing? The unit-count math for HUD and Agency loans

November 9, 2026

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Checked November 9, 2026. Program rules change. Confirm against the current documents linked below before you price a deal.

The first radon question on a multifamily deal is usually a number: how many units get a device? The answer depends on the program, and the wrong count shows up two ways. Under-scope, and the lender sends the report back. Over-scope, and you pay for tests nobody required.

The short answer. On a Federal Housing Administration (FHA) loan under HUD Multifamily Accelerated Processing (MAP), every ground-contact unit gets tested, plus a share of each upper floor. On a Freddie Mac loan, the Environmental Professional (EP) decides whether to test, and if so tests at least 10% of ground-contact units, with at least one test in every building that has ground-contact units. On a Fannie Mae loan, Fannie sets no radon sampling minimum. Scope follows federal, state or local law, and any lender requirement named for the project.

If you've seen "test 25% of ground-floor units" in an older article, that rule is gone from the current HUD, Fannie and Freddie documents.

First, what counts as a "ground-contact" unit

Counting starts with this definition, and most scoping mistakes happen here.

The ANSI/AARST multifamily test-location section says to measure in every occupied (or intended to be occupied) dwelling and non-residential room with floors or walls touching the ground. It also includes the lowest level of the building over a crawl space, utility tunnel, parking garage or other non-habitable space that is in contact with ground. It also says that where a ground-contact dwelling's lowest level could serve as living space, sleeping quarters, an office or a playroom, the test goes in that lowest level.

Two practical points:

  • Slab, basement and over-crawlspace units can all count. Don't count only the "first floor." Count what touches or sits directly over ground.
  • Freddie Mac's scope exemptions are a separate test. Freddie's Guide lists properties with no ground-contact residential units, such as units above a parking garage, as not requiring radon testing. The AARST protocol HUD's Guide points to lists the lowest occupied level over a parking garage, and the leasing office, as test locations. Same building, different answer, depending on the program. Scope to the program you're being underwritten under.

HUD MAP (FHA): all ground-contact units, then the upper floors

HUD's MAP Guide, Section 9.6.3.2.D, requires radon testing to follow the ANSI/AARST multifamily protocol and spells out the count: 100% of ground-floor units and 10% of upper-floor units, in all buildings in the project. HUD's Guide says "ground floor"; the protocol it points to defines that as ground-contact. The protocol's test-location section adds three details that move the number:

  1. Ground contact, every building. All dwellings and non-residential rooms that are occupied or intended to be occupied.
  2. Upper floors, per floor. On each upper floor, test at least one and not less than 10% of the dwellings and non-residential rooms that are occupied or intended to be occupied. Those tests are in addition to the ground-contact tests and in addition to any dwelling directly above an untested ground-contact location.
  3. Locations not to test. Unless it's for investigation, don't place devices in hallways, closets, or bathroom and shower areas unless they open into rooms occupied for other uses.

Two rules of thumb follow. The "at least one" floor minimum matters on small floors, where 10% rounds to less than one. And the 10% is figured per floor, in each building, not on a pooled property-wide total.

The Guide also lets an applicant skip the full assessment by mitigating all ground-level units and rooms in all buildings under the multifamily mitigation standard, and it allows a radon professional to document an exemption in a signed letter that cites the relevant standard. HUD staff decide whether to grant the exception.

Testing and any mitigation must be done by or under the direct supervision of a radon professional certified by the National Radon Proficiency Program (NRPP) or the National Radon Safety Board (NRSB), plus the state certification or license where the state requires one. The action level HUD uses is 4.0 picocuries per liter (pCi/L).

Freddie Mac: 10% of ground-contact units, minimum one per building

Freddie's Exhibit 11 (May 22, 2025) applies once the EP concludes testing is warranted. The EP must test or oversee testing of at least 10% of all ground-contact units at the property, chosen in the EP's professional judgment, with no fewer than one test in each building that has ground-contact units. Up to 15% of tests may be lost or faulty; above that, the EP decides whether the data is enough or more units need testing. Upper floors are not part of the Freddie minimum.

If the EP concludes testing isn't warranted, the reasoning goes in the executive summary of the environmental report.

Fannie Mae: no sampling minimum

Since Guide Update 25-07 (effective April 16, 2025), Fannie Mae's Form 4251 no longer includes a radon testing schedule. It asks the Phase I to cover radon testing required by federal, state or local law. Ask the lender what they want in writing before the quote.

Worked examples

These are illustrations of the rules above, not quotes, and not legal or lender advice. We rounded fractions up, because rounding down would put the count under 10%. Example B does not depend on rounding.

Example A: one four-story building, 16 units per floor, all ground-floor units on slab.

HUD MAPFreddie MacFannie Mae
Ground-contact units16 (100%)at least 2 (10% of 16 = 1.6, rounded up), min 1 per buildingNo Fannie minimum
Upper floors (3)at least 2 per floor (10% of 16 = 1.6, rounded up) = 6Not requiredNo Fannie minimum
Minimum test locations222EP / law / lender

Add any non-residential ground-contact rooms (leasing office, occupiable utility or storage rooms, maintenance rooms) to the HUD count. Add the HUD count for each building if the project has more than one. Add quality-control (QC) devices the protocol requires; confirm quantities in the standard.

Example B: six two-story buildings, four ground-floor units each (24 total).

HUD MAPFreddie Mac
Ground-contact units2410% of 24 = 2.4, rounded up to 3, but the one-per-building floor makes it 6
Upper floor (1 per building)at least 1 per building = 6Not required
Minimum test locations306

Example B is the one that surprises people. On a small-building, many-building property, the Freddie "10%" is really the "one per building" floor. The percentage never governs.

Example C: a mid-rise with a ground-floor leasing office and parking, and apartments starting on the second floor. Freddie's Guide lists properties with no ground-contact residential units, such as units above a parking garage, as not requiring radon testing. The AARST protocol HUD's Guide points to lists the lowest occupied level over a parking garage, and the leasing office, as test locations. On an FHA loan, plan to test them unless the radon professional documents an exemption and HUD grants it.

What changes the count after the first round

  • Lost or faulty devices. Under Freddie, more than 15% lost or faulty means the EP decides whether to place more.
  • Results at or above 4.0 pCi/L. Freddie lets the EP either recommend mitigation for each unit at or above 4.0, or retest only those units. HUD requires mitigation for units at or above its 4.0 pCi/L threshold, and post-mitigation testing under the mitigation standard.
  • Occupants not on board. The protocol requires prior notice to occupants and facilitating staff, and closed-building conditions during the test. A unit that can't be accessed or fails conditions doesn't produce a valid test, and the protocol allows only a few missing tests before more testing is needed. Plan access and notices before placement.

How to use this before you request a quote

  1. Name the program (HUD MAP, Freddie, Fannie Delegated Underwriting and Servicing (DUS)) and the lender's contact.
  2. Send the building count, the number of units and floors in each building, and which units touch or sit over ground.
  3. Say whether there are non-residential rooms (leasing office, maintenance, storage).
  4. Tell us the target closing date.

Request a quote through the form on this site or at quotes@multifamilyradontesting.com.

FAQ

How many apartments need radon testing on an FHA loan?

Under HUD's MAP Guide, 100% of ground-floor units and 10% of upper-floor units in every building in the project, following the ANSI/AARST multifamily protocol. Unless the radon professional documents an exemption that HUD grants, or the ground-level units are mitigated instead.

Is 25% of ground-floor units still the rule for Fannie Mae or Freddie Mac?

No. Freddie Mac's minimum is now 10% of ground-contact units with at least one test per building that has ground-contact units, once the EP decides testing is warranted. Fannie Mae's Form 4251 no longer includes a radon testing schedule.

What is a ground-contact unit?

A dwelling with floors or walls in contact with the ground, including the lowest occupied level over a crawl space, utility tunnel or parking garage under the AARST protocol, plus occupied ground-contact rooms such as a leasing office. Freddie's Guide lists properties with no ground-contact residential units, such as units above a parking garage, as not requiring radon testing.

Who decides how many units to test on a Fannie Mae loan?

Fannie sets no radon sampling minimum. Scope follows federal, state or local law, and any lender requirement named for the project. Since Guide Update 25-07 (effective April 16, 2025), Fannie Mae's Form 4251 no longer includes a radon testing schedule. It asks the Phase I to cover radon testing required by federal, state or local law. Ask the lender what they want in writing before the quote.

Who can perform the testing?

HUD requires a radon professional with NRPP or NRSB certification plus state certification or license where required. Freddie has the Environmental Professional manage the testing, and state certification applies where state law requires it.

Do upper floors need testing for Freddie Mac?

Freddie's Exhibit 11 minimum is about ground-contact units. It does not set an upper-floor requirement. HUD's does.

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