The core stress test works as follows: the lender calculates what the monthly mortgage interest would be at a stressed interest rate (typically 5.5% or higher, regardless of the actual product rate), then divides the monthly rental income by that stressed interest figure. The result must meet the lender's minimum ICR — usually 125% for basic rate taxpayers or those using a limited company structure, and 145% for higher or additional rate taxpayers. If the property does not pass the ICR test at the required level, the borrower must either reduce the loan amount, find a property generating higher rent, use a lender offering top-slicing, or consider the limited company route.
For portfolio landlords — those with four or more mortgaged buy-to-let properties — the PRA's 2017 rules require lenders to assess the full portfolio, not just the property being mortgaged. This means every existing mortgaged BTL in the portfolio must be reviewed and stress tested, typically requiring specialist buy-to-let underwriting rather than standard automated processing. Portfolio landlords are often directed to specialist lenders or specialist divisions of mainstream lenders who have the infrastructure to conduct this analysis.
ICR requirements, stressed rate calculation and the s.24 higher-rate adjustment
How buy-to-let mortgage affordability is assessed and why higher-rate taxpayers face a tighter test:
- Interest coverage ratio (ICR) — the core stress test: The ICR is the ratio of monthly rental income to monthly mortgage interest at the lender's stressed rate. Formula: ICR = Monthly Rent ÷ (Loan Amount × Stressed Rate ÷ 12). The ICR must equal or exceed the lender's minimum threshold. Stressed rate: lenders typically stress at 5.5% (the minimum recommended by PRA guidance) or at the higher of: (a) the lender's standard stressed rate (commonly 5.5%); or (b) the initial product rate plus 2% (applied where the product rate is already high). Example: £200,000 BTL mortgage; stressed rate 5.5%; monthly stressed interest = £200,000 × 5.5% ÷ 12 = £916.67; at 125% ICR: rent must be at least £916.67 × 1.25 = £1,145.84/month; at 145% ICR: rent must be at least £916.67 × 1.45 = £1,329.17/month. Why 125% vs 145%? The ICR threshold reflects the landlord's tax position. For a basic rate taxpayer (or limited company), the mortgage interest cost can be recovered from rental income with relatively low tax leakage. For a higher or additional rate taxpayer subject to Section 24 (the 20% tax credit restriction), a portion of the mortgage interest cannot be deducted — meaning a higher ICR is needed for the property to be cashflow-sustainable after tax. Most lenders apply: (a) 125% ICR for basic rate taxpayers; (b) 145% ICR for higher rate taxpayers; (c) 125% ICR for limited company borrowers (companies are not subject to s.24). Lender variation: ICR requirements and stressed rates vary by lender — always obtain a Decision in Principle to understand the specific lender's criteria
- Top-slicing, EPC requirements and additional lender criteria: Top-slicing: where the rental income alone does not generate sufficient ICR, some lenders will 'top-slice' — allowing the borrower's personal income (salary; pension; other rental income) to supplement the rental income shortfall when calculating affordability. Not all lenders offer top-slicing; those that do will have their own income requirements and documentation standards (typically payslips; P60; SA302; accountant's letter). Top-slicing is more common for portfolio landlords and professional property investors. EPC requirements: many lenders now require the property to have an EPC rating of D or above (minimum E technically required by current MEES regulations, but lenders anticipate the 2030 EPC C requirement and many now require D+). Some lenders require EPC C for new BTL mortgage applications, particularly on green mortgage products. Lease length: for leasehold properties, lenders typically require a minimum lease length of between 70 and 85 years remaining at the end of the mortgage term — a 25-year mortgage on a leasehold flat would therefore typically need 95-110 years minimum remaining lease at mortgage start (check individual lender criteria). Flood risk: some lenders apply restrictions in designated flood risk zones or require flood insurance evidence. HMO mortgages: specialist BTL products — most standard BTL lenders will not lend on HMOs; specialist lenders apply different ICR criteria (often 130% or higher); HMO BTL may require planning confirmation and HMO licence
Portfolio landlord underwriting (PRA 2017), limited company BTL and devolved positions
The specialist assessment required for landlords with four or more mortgaged properties and how limited company structures affect the stress test:
- PRA 2017 portfolio landlord rules — four or more mortgaged properties: PRA Supervisory Statement SS13/16 (updated September 2017) introduced additional underwriting requirements for 'portfolio landlords' — defined as those with four or more mortgaged buy-to-let properties across all lenders (not just with the lender being approached). Portfolio landlord criteria: (a) four or more mortgaged BTL properties at any point (including the proposed new mortgage); (b) properties may be held personally or in limited companies; (c) total count is across all lenders — a landlord with 2 mortgages at lender A and 2 at lender B is a portfolio landlord when approaching lender C for a 5th. For portfolio landlords, lenders must: (i) collect information on the full portfolio (all properties, values, outstanding mortgage balances, rental income, existing mortgage payments); (ii) stress test the entire portfolio, not just the property being mortgaged; (iii) review the landlord's overall financial position including personal income, liabilities, and business plan; (iv) apply specialist underwriting rather than automated approval. Practical impact: portfolio landlord applications take longer (2-6 weeks typical versus days for standard BTL); require more documentation (portfolio spreadsheet; last 2-3 years of accounts if company; SA302s; asset and liability statement); may be declined by mainstream lenders who lack specialist BTL teams — directing portfolio landlords to specialist lenders (e.g., Paragon; Foundation; Aldermore; Fleet; specialist divisions of NatWest; BM Solutions). Background properties: even background properties that pass individually must collectively demonstrate a sustainable portfolio — the lender assesses the overall portfolio ICR, not just individual property metrics
- Limited company BTL, Scotland, Wales and NI: Limited company BTL mortgages: a buy-to-let mortgage taken by a Special Purpose Vehicle (SPV) limited company (typically a company set up solely to hold and let property — SIC code 68100 or 68209). Key differences from personal BTL: (a) ICR: most lenders apply 125% ICR for limited company BTL (lower than 145% for HR taxpayer individuals) — the company pays corporation tax on profits; interest is deductible for the company (no s.24 restriction); (b) mortgage rates: typically 0.1-0.3% higher than equivalent personal BTL products; (c) personal guarantee: lenders typically require personal guarantees from company directors/shareholders; (d) lender choice: growing market but still fewer lenders than personal BTL; specialist lenders are key; (e) valuation: same as personal BTL — RICS valuation; rental income evidence. Incorporation of existing personal portfolio: transferring personally-held properties into a limited company triggers SDLT (and LBTT in Scotland; LTT in Wales) on the market value of each property — major cost; seek tax advice before incorporating an existing portfolio. Scotland: same PRA regulatory framework applies; Scottish lenders (and UK lenders lending in Scotland) must comply with SS13/16; LBTT applies on purchase; no stamp duty. Wales: same PRA framework; LTT applies on purchase. NI: same PRA framework; SDLT applies on purchase. Regional lenders: some Scottish building societies (e.g., West Bromwich; Skipton for Scotland; Clydesdale) have different BTL criteria; always check with a specialist BTL broker for current products in your region
Frequently asked questions
What is the minimum ICR required for a buy-to-let mortgage in 2026?+
Most lenders require a minimum interest coverage ratio (ICR) of 125% for basic rate taxpayers and limited company borrowers, and 145% for higher or additional rate taxpayers. The ICR is calculated using a stressed interest rate — typically 5.5% or higher, regardless of the actual product rate — to ensure affordability if rates rise. For example, a £150,000 BTL mortgage at a 5.5% stressed rate requires monthly rent of at least £859 (125% ICR) or £997 (145% ICR). Individual lenders may apply different thresholds — always check the specific criteria with the lender or a BTL mortgage broker.
Am I a portfolio landlord for mortgage purposes and does it affect my application?+
The PRA defines a portfolio landlord as someone with four or more mortgaged buy-to-let properties (across all lenders, not just the lender you are approaching). If you own 4+ mortgaged BTL properties and apply for a new mortgage, the lender must conduct specialist portfolio underwriting — assessing your entire portfolio (all properties, values, mortgages, and rental income) not just the new property. This typically requires more documentation (a portfolio spreadsheet, full accounts if using a company, SA302s, and an asset and liability statement) and takes longer than standard BTL applications. Many mainstream lenders direct portfolio applications to specialist divisions or specialist lenders.
Does using a limited company reduce the BTL mortgage stress test threshold?+
For many lenders, yes. Most lenders apply a 125% ICR to limited company (SPV) buy-to-let mortgages rather than the 145% applied to higher-rate taxpaying individuals. This is because limited companies are not subject to Section 24 (the mortgage interest tax credit restriction) — the company deducts mortgage interest in full before calculating corporation tax, so the net income position after tax is more favourable than for a higher-rate individual landlord. However, limited company BTL mortgage rates are typically 0.1-0.3% higher, and personal guarantees are required. The benefit of a lower ICR threshold must be weighed against higher product rates and the additional administration of running a company.
Can my personal income be used to top-up a failing BTL mortgage stress test?+
Some lenders offer 'top-slicing' — allowing personal income (salary, pension, or other income) to be used to make up any shortfall where the rental income alone does not meet the ICR test. Not all lenders offer top-slicing; those that do will have their own income thresholds and documentation requirements. Top-slicing is most commonly available at specialist BTL lenders and through specialist broker channels. It can be useful where a property generates slightly below the required rent but the landlord has strong personal income. Check with a specialist BTL mortgage broker to identify lenders who offer top-slicing for your profile.
- Buy-to-let mortgages — full guide to products, criteria and costs →
- Limited company buy-to-let — tax benefits, costs and mortgage access →
- Section 24 — mortgage interest tax restriction for individual landlords →
- Portfolio refinancing — remortgaging multiple BTL properties →
- Interest-only BTL mortgages — criteria and repayment strategies →
- Remortgaging a buy-to-let — when and how to switch product →