Portable Mortgages Sound Great. The Math Is Messier.

There is a reason H.R. 10028 is getting attention.

It speaks directly to one of the biggest frustrations in housing today: homeowners who are effectively locked into their current homes because the mortgage they already have is far more attractive than the mortgage they would have to take on if they moved.

It’s a real problem affecting household mobility, inventory turnover, and the willingness of would-be move-up buyers to make a change even when life is pushing them in that direction.

So when Representative Tom Kean Jr. introduced H.R. 10028, the MOVE Act, the headline was naturally compelling. The pitch is simple enough for a cocktail conversation: what if a homeowner could take their mortgage with them when they move?

At first glance, that sounds like a breakthrough. If someone has a $500,000 mortgage at 3.00%, why should they be forced to give that up just because they want to buy a different house? Why not let them transfer that loan balance, rate, and remaining term to the next property?

It is a smart political message because it sounds like common sense. It also happens to leave out some very important details.

The first and most important issue is that the bill does not appear to do what many casual listeners will assume it does. H.R. 10028 does not say that every existing low-rate mortgage suddenly becomes portable. It directs Fannie Mae and Freddie Mac to begin purchasing and securitizing conventional mortgages that are designed to allow portability. That is a very different idea.

In plain English, this looks much more like a proposal for a future mortgage product than a retroactive rescue plan for today’s 3% borrowers.

That distinction matters.

A homeowner hearing the phrase “transfer your existing mortgage rate, term, and balance to a new property” could easily assume Congress is trying to let them keep the mortgage they already have. But the economics of the mortgage market make that far more complicated. Existing low-rate mortgages are not simply sitting on a bank’s balance sheet waiting to be modified out of generosity. In most cases, those loans have already been sold into the secondary market and are owned, directly or indirectly, by investors who purchased cash flows based on a certain set of assumptions. One of those assumptions is that when the borrower sells the home, the loan generally gets paid off.

Portable mortgages change that.

That is the overlooked angle here. Portability is not just a consumer perk. It is a borrower option, and options have value. If a borrower can keep a below-market mortgage even after selling the home, the investor loses one of the most common paths to getting principal returned and redeployed at current yields. If market rates are 6.75% or 7.00% and the investor is stuck collecting 3.00% for years longer than expected, that is not a small issue. It is the entire pricing issue.

Which is why portability is not free.

Analysts looking at portable mortgages have already suggested that borrowers would likely pay for that flexibility upfront, probably in the form of a somewhat higher interest rate at origination. In other words, if portable mortgages become a standard future product, the borrower may be buying the right to preserve that mortgage later. Useful, yes. Free, no.

Even that, however, is only the first layer of the story.

The second layer is the one I think gets missed almost entirely in the public conversation: portability helps only on the portion of the new purchase that can be covered by the old mortgage balance. It does not solve the rest of the transaction.

Let’s walk through a realistic example.

Suppose a homeowner sells a current home for $1.2 million. They have an existing first mortgage of $500,000 at 3.00%. On paper, that leaves about $700,000 of gross equity. But nobody gets to move “gross equity” into the next house. Selling costs come first. Between commissions, escrow, title, transfer charges, and other transaction costs, it would not be hard to see $60,000 to $70,000 disappear before the homeowner ever touches the proceeds.

So now the net equity available is closer to $630,000 to $640,000.

Assume that homeowner then buys a new home for $1.6 million. If the $500,000 mortgage is portable, great. That old loan balance moves over. The homeowner also applies roughly $630,000 to $640,000 in net proceeds from the sale. But the new purchase price is still $1.6 million.

The gap does not vanish.

At that point, the borrower still needs roughly $460,000 to $470,000 in additional financing.

That is where the clean political story starts to get messy.

The homeowner is no longer financing the new purchase at 3.00%. They are financing part of it at 3.00% and part of it at whatever the market demands for the new money. And because that new money may need to come in as a second lien or other subordinate structure, the rate on that gap financing may be meaningfully higher than the rate on a standard first mortgage.

Now we are talking about the actual capital stack, not the campaign version.

Let’s use $465,000 as the new financing amount. If that money potentially carries an 8.10% rate (or higher), which is not unreasonable for higher-risk secondary financing, the borrower’s balance-weighted blended rate would look attractive on paper. You would have $500,000 at 3.00% and $465,000 at 8.10%, producing a rough blended rate of about 5.46%.

That sounds pretty good relative to financing the full amount at today’s first-mortgage rates.

But rate is only part of the story. Payment matters more.

The old portable mortgage would not magically become a fresh 30-year loan. If that mortgage has 24 years remaining, then that is likely the remaining term coming into the new property. If the additional $465,000 is financed separately at 8.10%, the amortization period on that second piece becomes crucial.

If the $500,000 portable first has 24 years remaining at 3.00%, the principal and interest payment is about $2,438 per month. If the $465,000 gap financing is set up at 8.10% over 20 years, the principal and interest payment is about $3,918 per month. Combined, the borrower is at roughly $6,356 per month in principal and interest.

Now compare that with a single new $965,000 mortgage at 6.75% over 30 years. That payment would be roughly $6,259 per month.

Read that again. The borrower “kept the 3% mortgage,” and the monthly payment still comes out slightly higher in this structure than simply taking one new 30-year first mortgage at current rates.

That is not because portability has no value. It absolutely can. It is because the value of the portable piece can be weakened or even overwhelmed by the structure, pricing, and amortization of the gap financing.

And if the second piece is amortized more gently over 30 years instead of 20, the payment improves. In that case, the $465,000 loan at 8.10% would be about $3,444 per month, bringing the combined payment to around $5,882. Now portability is helping more clearly. But even then, the borrower still has to manage a more complex financing structure, potentially with two liens, two different terms, and the complications that come with that.

This is the part of the conversation that deserves more attention.

Portable mortgages are easy to understand at the slogan level. They are much harder to evaluate in an actual transaction.

Who underwrites the gap financing? Does it come in behind the portable first as a true second lien? What does that do to combined loan-to-value limits? How is the borrower requalified? What happens if the new property is a condo with its own approval issues? What happens if values soften and the subordinate lender becomes more conservative? What if the second lien is adjustable, or requires a shorter payoff horizon? What if the borrower later wants to refinance one piece but not the other?

Those are not side questions. Those are the deal.

That is why I think the real story around H.R. 10028 is not that portable mortgages are a bad idea. In concept, they are actually a very interesting idea. They recognize that a mortgage is not just debt; under the right circumstances, it can also be a valuable financial asset. A homeowner sitting on a 3.00% loan in a 7.00% world clearly owns something economically valuable.

The problem is that Washington has a habit of selling the headline benefit of a financing innovation while understating the cost, complexity, and tradeoffs underneath it.

We have seen versions of this before. A 40-year mortgage can be pitched as a lower-payment solution, but extending duration changes the economics of the loan and the way investors price the risk. The payment may go down relative to a shorter term, but the borrower pays for longer, builds equity more slowly, and can end up carrying more interest over time. The wrapper changes. The economic realities do not disappear.

Portable mortgages raise a similar issue. If you give borrowers a valuable option, someone has to absorb that cost. If you preserve only part of a low-rate financing structure, someone still has to fund the rest. And when that additional capital sits in a riskier position, it usually commands a higher rate.

That is the story.

The MOVE Act, authored by Representative Tom Kean Jr., is politically sharp because it taps into a genuine frustration in the housing market. Homeowners feel trapped by the success of their old financing. They know their current mortgage is too good to casually surrender. Any policy proposal that seems to honor that reality is going to attract attention.

But attention and execution are two different things.

If portable mortgages eventually become a viable conventional product, they may help mobility for some borrowers. They may improve transaction flow. They may allow some homeowners to preserve a meaningful portion of their financing advantage when they move. That would be a real benefit.

What they will not do is magically turn a move-up purchase into a 3.00% financing event. They will not erase selling costs. They will not eliminate the need for new capital. And they will not prevent the market from pricing the risk associated with that structure.

The cleanest way to say it is this: portability may preserve the cheap money you already have, but it does not make the next house cheap.

For borrowers, advisors, and anyone trying to think clearly about housing policy, that is the conversation worth having. Not whether portable mortgages sound good. They do. The better question is whether the full financing stack still makes sense once the math, structure, and incentives are laid out honestly.

That is where the real answer lives.