Beyond the Guide: A Deep Dive into Mortgage Underwriting - Adjustable Rate Mortgage Qualification Refresher
When rates climb, the phone calls start: “Is there anything with a lower payment?” According to Freddie Mac’s Primary Mortgage Market Survey, the 30-year fixed rate averaged 7.40% as of October 8, 2026, up from 7.28% the week before and 6.30% a year ago. That is more than a full point in twelve months, and for many buyers it is the difference between qualifying and not qualifying.
That is when the adjustable rate mortgage (ARM) comes back off the shelf. A lower initial rate can mean a lower initial payment, and a lower payment can mean a lower DTI. But an ARM is not just a fixed-rate loan with a discount. It has its own eligibility rules, its own qualifying rate, and its own disclosure requirements, and each one is a place where a file can go wrong. If your team has not underwritten many ARMs since rates were low, this post is your refresher.
Underwriting guidelines change rapidly. For the most up-to-date information, please refer to the Fannie Mae Selling Guide at Selling Guide | Fannie Mae or for Freddie Mac at Guide Home. This post focuses on Fannie Mae requirements. Freddie Mac has its own ARM requirements, so check its Guide separately.
Step One: Is It an Eligible ARM?
Before anyone calculates a qualifying payment, the ARM itself has to fit the box. B2-1.4-02, Adjustable-Rate Mortgages (ARMs) sets out the characteristics, and the details matter:
Initial fixed period: The initial adjustment period must match the initial fixed-rate period: 36 months for a 3-year ARM and 60 months for a 5-year ARM.
Index: Plans must be tied to SOFR. Fannie Mae uses a 30-day average of the SOFR index published daily by the Federal Reserve Bank of New York.
Margin: The maximum mortgage margin may be no more than 300 basis points, and the rate may never fall below the margin, even with a lower cap.
Caps: Each plan must have lifetime and per-adjustment limits. Lifetime limits apply to rate increases only. Per-adjustment limits apply to both increases and decreases.
Fully indexed rate: The index plus the margin, rounded to the nearest one-eighth percent. Any index value in effect during the 90 days before the note date may be used.
Rate changes: The new rate equals the margin plus the most recent index value available 45 days before the change date, rounded to the nearest one-eighth, and still subject to the caps.
Initial note rate limit: The Guide limits how far the initial note rate can sit below the fully indexed rate. For 3- and 5-year ARMs, the difference must not exceed 3%.
Plan number: Lenders must identify the applicable Fannie Mae ARM plan number in the closing documents and at delivery.
Two more items often get missed. If the plan includes a conversion option, the disclosures must cover when conversion is allowed, the request and return deadlines, and the fees. And if a temporary buydown is used with an ARM, the property must be a principal residence or second home, and the plan must have an initial interest rate period of at least 3 years.
Step Two: Qualify at the Right Rate
This is where ARM files most often go sideways. Borrowers on a fixed-rate loan qualify at the note rate. Borrowers on an ARM do not always qualify at the note rate, because the lender must protect against payment shock. The rules are in B3-6-04, Qualifying Payment Requirements, and they depend on the initial fixed-rate period:
Three years or less: Qualify at the maximum interest rate that could apply during the first five years after the first payment is due.
Five years: Qualify at the greater of the maximum rate that could apply during the first five years after the first payment date (the note rate plus the first rate change cap) or the fully indexed rate.
Greater than five years: Qualify at no less than the note rate. For higher-priced mortgage loans or higher-priced covered transactions under Regulation Z, use the greater of the note rate or the fully indexed rate. If the fully indexed rate is higher, the loan must be manually underwritten.
For loans run through Desktop Underwriter, the Guide also spells out the qualifying rate for the generic ARM plans:
1-year ARM (1% annual cap): Note rate plus 5%
1-year ARM (2% annual cap): Note rate plus 6%
3-year ARM: Note rate plus 5%
5-year ARM: Greater of the fully indexed rate or note rate plus 2%
7- and 10-year ARM: Note rate
Lender ARM plans: Rate entered in the ARM Qualifying Rate field in DU (note rate plus 5% if blank)
Temporary buydowns do not change the math. Loans subject to a temporary interest rate buydown must be qualified without consideration of the bought-down rate.
The payment calculated at the qualifying rate is the payment that goes into the DTI ratio. That is why a 5-year ARM with an attractive note rate can still produce a higher qualifying payment than a fixed-rate loan, as the example below shows.
Putting Numbers to It: What the Qualifying Payment Really Looks Like
The example below is hypothetical. It assumes a $400,000 loan, a 30-year amortization, and principal and interest only. The fixed rate uses the Freddie Mac weekly average. The ARM note rates are illustrative, and the 5-year ARM assumes a 2% first adjustment cap and a fully indexed rate of 7.00%.
30-year fixed: note rate 7.40%, qualifying rate 7.40%, P&I at note rate $2,769.52, P&I at qualifying rate $2,769.52.
5-year ARM: note rate 6.50%, qualifying rate 8.50%, P&I at note rate $2,528.27, P&I at qualifying rate $3,075.65.
7-year ARM: note rate 6.75%, qualifying rate 6.75%, P&I at note rate $2,594.39, P&I at qualifying rate $2,594.39.
3-year ARM: note rate 6.25%, qualifying rate 11.25%, P&I at note rate $2,462.87, P&I at qualifying rate $3,885.05.
Three takeaways:
The 5-year ARM saves the borrower money but not DTI. The note payment is about $241 a month lower than the fixed-rate payment. But the qualifying payment is about $306 a month higher, because the loan is qualified at note rate plus 2%.
The 7-year ARM is the DTI winner. Qualifying at the note rate means the lower payment flows straight into the ratio, subject to the higher-priced mortgage loan rule noted above.
The 3-year ARM rarely works in a high-rate environment. Qualifying at note rate plus 5% pushes the payment well above even the fixed rate.
The point for loan officers: the ARM that saves the most on the note rate is not necessarily the one that helps the borrower qualify. The qualifying rate decides that.
Disclosures and Where the Defects Happen
ARMs carry more paper than fixed-rate loans. The Guide expects disclosures about the availability of index values and, where a conversion option exists, about the conversion terms and fees. Beyond the Selling Guide, lenders also have the federal ARM disclosure requirements under Regulation Z and the adjustable payment information on the Loan Estimate and Closing Disclosure to get right. Confirm those against your compliance team’s current requirements.
A QC review of ARM files should test each of these:
Wrong qualifying rate. The note rate was used on a 3-year or 5-year ARM that required a higher qualifying rate.
Fully indexed rate miscalculated. The index is not the SOFR average, the rounding to the nearest one-eighth is wrong, or the index value used falls outside the 90 days before the note date.
Margin or floor problems. The margin exceeds 300 basis points, or the rate floor sits below the margin.
Initial note rate too far below fully indexed. For a 3-year or 5-year ARM, the difference exceeds the 3% limit.
Initial adjustment period mismatch. The first adjustment does not line up with the initial fixed-rate period.
Buydown qualified at the bought-down rate. The qualifying payment should ignore the temporary buydown.
Higher-priced loan not tested. On ARMs with an initial fixed period longer than five years, the file does not show whether the loan is higher priced and whether the fully indexed rate requires manual underwriting.
Plan number missing. The Fannie Mae ARM plan number is not identified in the closing documents or at delivery.
Conversion terms undisclosed. The plan has a conversion option and the disclosures omit the timing, deadlines, or fees.
The “Why” Behind the ARM Refresher
Why review rules you may have used years ago? Because when rates rise, ARM volume rises with them, and the qualifying rules that kept payment shock in check are exactly what gets skipped when a team is out of practice. A loan qualified at the wrong rate is a DTI the file cannot support, and that is the kind of defect that leads to “buy-back” risk on the secondary market. An ARM can be a smart option for the right borrower. It just has to be underwritten like one.
The Commonwealth Group: Precision. Integrity. Results.
When volume shifts to products your team has not seen in a while, the right partner makes the difference. At The Commonwealth Group, our contract underwriting and quality control teams know the ARM rules, check the qualifying rate, and verify the plan details so the loan you close is the loan you can sell. We do not just follow the guide; we know what to do when the guide is not enough.
Ready to go beyond the guide?
Contact Martin Luplow at The Commonwealth Group – info@thecommonwealth.net for more information on what our team of professionals in processing and underwriting can do for your company.
The Commonwealth Group – Innovative Services for the Mortgage Industry
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