Somewhere in the small Swiss city of Basel, inside a circular tower overlooking the Rhine, a committee of central bankers has been quietly reshaping the rules of global finance for half a century. What they’ve decided is now about to land squarely on the UK lending market, and if you’re advising clients on mortgages, you need to understand what’s coming.
Basel 3.1 is the PRA’s final overhaul of UK bank capital rules, taking effect on 1 January 2027, with some market risk internal model changes deferred to 1 January 2028. Its main purpose is to reduce the variability created by internal models, apply more granular LTV-based mortgage risk weights, and constrain the capital advantage that large IRB banks can gain over lenders using simpler approaches. A transitional period runs through to 1 January 2030.
Some of those changes will be subtle. Others won’t be. To understand where we’re going, it helps to understand how we got here.
A bank failure in Cologne
The story starts not with the 2008 financial crisis, as many assume, but in 1974. On 26 June that year, German regulators forced a mid-sized bank called Bankhaus Herstatt into liquidation. Herstatt had been trading foreign exchange and had built up enormous positions it couldn’t cover. When the regulators pulled the plug at 4:30pm German time, it was only 10:30am in New York. Banks that had already paid Deutsche Marks to Herstatt were waiting for US dollars to arrive on the other side of the Atlantic. Those dollars never came.
The fallout was immediate and international. Counterparty banks were left holding losses they hadn’t anticipated, and the episode exposed a basic truth: banking had become global, but banking supervision hadn’t. No single regulator had visibility across borders. There was no shared rulebook, no common standards, no mechanism for cooperation.
By the end of 1974, the central bank governors of the G10 nations had created the Basel Committee on Banking Supervision, housed at the Bank for International Settlements in Basel. Its first meeting was in February 1975, and one of its first major outputs was the Concordat, a simple but radical principle: no internationally active bank should escape supervision in any jurisdiction where it operates.
From Concordat to capital rules
For the first decade, the Committee focused on supervisory cooperation rather than capital rules. But by the mid-1980s, cooperation alone wasn’t enough. Banks were growing rapidly, taking on more risk, and the amount of capital they held varied wildly between countries. A bank in Tokyo and a bank in London could hold completely different amounts of capital against identical lending portfolios.
Basel I, published in 1988 and enforced by 1992, was the first attempt to fix this. It required banks to hold capital equal to at least 8% of their risk-weighted assets, focused solely on credit risk. It was blunt and imperfect, but it created a common language that regulators across the world could work from.
Basel II arrived in 2004 and introduced a critical innovation: it allowed larger banks to use their own internal models to calculate risk weights, rather than relying solely on standardised regulatory formulas. In theory, a bank with decades of granular lending data should be better at assessing risk than a one-size-fits-all formula. In practice, internal models often produced lower risk weights and therefore lower capital requirements, creating a significant competitive advantage for the biggest banks over smaller lenders using the standardised approach. That model variability under Basel II is the thread that runs all the way through to Basel 3.1.
Then came the crash
The 2008 financial crisis tore the roof off. Banks that had appeared well-capitalised under Basel II turned out to be holding far less genuine loss-absorbing capital than anyone realised. Risk models that had been approved by regulators proved wildly optimistic. Leverage had spiralled. Liquidity buffers were thin.
Basel III, published in 2010 and 2011, was the direct response. It raised the minimum common equity requirement from 2% to 4.5% of risk-weighted assets, introduced a leverage ratio as a backstop, tightened the definition of what counted as capital, and added liquidity requirements for the first time. The intention was clear: never again.
But Basel III still left one problem unresolved. Those internal models were still producing widely varying risk weights across different banks for essentially the same exposures. Two banks could lend to the same borrower, against the same property, and arrive at materially different capital requirements. That variability undermined confidence in the entire framework.
Basel 3.1 finishes the job.
Enter Basel 3.1
Agreed internationally in 2017, Basel 3.1 has taken the best part of a decade to translate into national rules. The PRA published its final rules for the UK in January 2026 (PS1/26), with the main capital regime taking effect on 1 January 2027. The market risk internal model approach (known as FRTB-IMA) is deferred by one year to 1 January 2028, giving firms additional time to prepare while allowing other Basel 3.1 improvements to bed in during 2027.
The headline change is the introduction of an output floor. Banks using internal models to calculate risk weights will now be required to hold capital equal to at least 72.5% of what the standardised approach would require. That floor is phased in over the transitional period, reaching the full 72.5% by the end of 2030.
The effect is significant. A large bank whose internal models currently risk-weight a portfolio of residential mortgages well below the standardised level will find those models constrained. The output floor means the capital benefit from sophisticated modelling is capped. You can still use your models, but they can’t take you more than 27.5% below what the standardised formula would demand.
For smaller banks and building societies that already use the standardised approach, this is potentially good news. It levels a playing field that has been tilted in favour of the largest banks for two decades.
What it means for mortgage lending
This is where it gets practical.
Under the new standardised approach, the PRA has introduced more granular LTV-based risk weights for residential mortgages. Lower LTV loans attract lower risk weights, with the scale becoming progressively more punitive as LTV increases. That’s a much more risk-sensitive framework than the current one, and it means lenders will need to think carefully about the capital cost of different parts of their mortgage book.
Buy-to-let is where the biggest shift lands. The PRA reviewed the evidence the industry presented and did introduce more granular risk weights for medium-LTV BTL lending, which was a welcome concession from the original proposals. But the overall direction of travel is clear: BTL risk weights are going up. Industry projections suggest capital requirements for BTL books could increase materially, with some estimates pointing to a potential reduction in lending capacity or upward pressure on pricing. These are industry projections rather than PRA conclusions, but they give a sense of the direction. Professional portfolio landlords are likely to feel the effects most acutely, whether through reduced product availability, tighter criteria, or higher borrowing costs.
There are also new valuation requirements. Lenders must revalue properties used as collateral at least every five years, with a shorter cycle in certain higher-risk cases. If the broader property market falls by 10% or more, revaluation becomes mandatory regardless of the schedule. The PRA has confirmed that Automated Valuation Models will still be permitted for this purpose, which should keep costs manageable for most lenders.
Self-build mortgages don’t get the exemptions some had hoped for. The PRA decided that carving them out wouldn’t be proportionate to the risk, so they’ll be subject to the standard framework with adjusted valuations.
Building societies face a particular challenge, though not because of a new Basel rule. The existing requirement that at least 75% of their lending be secured against residential property means they carry a structural concentration in exactly the asset class where risk weights are changing. Any increase in capital requirements for residential mortgages hits them harder than diversified banks with large corporate and commercial books. It’s an exposure concentration issue rather than a new regulatory burden, but it makes Basel 3.1 a bigger deal for societies than for most banks.
Are there benefits?
It would be easy to frame Basel 3.1 as just another regulatory burden. But there are genuine benefits worth acknowledging.
Financial stability is the obvious one. The reforms address real weaknesses that the 2008 crisis exposed. More consistent risk weighting across the banking system means more confidence that capital ratios actually reflect the risks being taken. That matters for everyone, including borrowers.
The levelling effect of the output floor matters too. For years, the largest banks have enjoyed a capital advantage through their internal models, making it harder for mid-tier lenders and building societies to compete on price. By constraining that advantage, Basel 3.1 should, in theory, create a more competitive lending market. Whether that translates into better outcomes for borrowers and brokers remains to be seen, but the structural logic is sound.
The PRA has also introduced a Strong and Simple framework alongside Basel 3.1, designed specifically for smaller banks and building societies. It simplifies the calculation requirements, reduces the compliance burden, and introduces a single consolidated capital buffer instead of the complex “buffer stack” that currently applies. That’s a meaningful step towards proportionate regulation, and it should give smaller lenders more room to grow and compete.
The PRA’s own estimate is that the aggregate increase in Tier 1 capital requirements for major UK firms will be less than 1%. That’s a carefully managed landing, and it reflects a regulator that has listened to industry concerns and calibrated accordingly.
The view from here
For mortgage professionals, the practical implications of Basel 3.1 will unfold gradually rather than arriving as a single shock. The transitional period to 2030 gives lenders time to adjust their books, and the PRA has clearly tried to avoid a cliff edge.
But the direction is clear. Capital requirements for BTL lending are going up. Building societies will need to think harder about capital efficiency. The competitive dynamics between large banks using internal models and smaller lenders on the standardised approach will shift. And every lender will be running the numbers on how these changes affect their product pricing and risk appetite.
If you’re advising clients, particularly landlords and those at higher LTVs, understanding the capital backdrop to lender pricing decisions becomes more important than it has been for years. The rates your clients see on their screens in 2027 and beyond won’t just reflect base rate expectations and swap rates. They’ll reflect the capital cost of the loan to the lender, and Basel 3.1 is about to change that calculation.
Fifty years after a bank failure in Cologne sent shockwaves across the Atlantic, the regulatory architecture that event set in motion is still being refined. Basel 3.1 isn’t the end of the story. But it is, hopefully, the closing chapter of the post-crisis reform programme that began in 2008. For those of us in the mortgage market, the question isn’t whether it will affect us. It’s how well we understand it when it does.
Sources:
– Bank of England — PS1/26: Implementation of Basel 3.1 Final Rules
– UK Finance — The PRA’s Big Reveal: A Balanced Approach to Finishing Basel 3.1
– Bank for International Settlements — History of the Basel Committee
– Mortgage Solutions — PRA Publishes Basel 3.1 Statement with Updated Mortgage Lending Rules
– PwC — PRA Publishes Final Basel 3.1 Rules
– Finadium — UK Regulator Consults on Basel 3.1 with FRTB Delayed to 2028