Three building standards, one house size: 25 years of mortgage and heating costs compared

• Area: all three houses use the single floor area entered above — this is a like-for-like comparison of standards, not of house sizes.
• Space heating demand: calculated in PHPP (Passive House Planning Package) for each house. These figures come from the modelled PHPP files, not universal outcomes for each standard. All three houses are air tested on completion, as required for new homes in Northern Ireland; only House C’s design and construction are also independently certified.
  • House A 64 kWh/m²/yr (typical Building Regulations-compliant house): airtightness 4.0 ACH (n₅₀ at 50 Pa). Double glazing: Ug = 1.20 W/m²K, g-value = 0.50, Uf = 1.40 W/m²K, glazing-edge psi-value = 0.04 W/mK, window Uw-value = 1.4 W/m²K. Roof U-value = 0.16 W/m²K, wall U-value = 0.18 W/m²K, floor U-value = 0.18 W/m²K. No MVHR (traditional extract ventilation).
  • House B 40 kWh/m²/yr (“Passive House principles”): airtightness 3.0 ACH (n₅₀ at 50 Pa). Double glazing: Ug = 1.20 W/m²K, g-value = 0.50, Uf = 1.40 W/m²K, glazing-edge psi-value = 0.04 W/mK, window Uw-value = 1.4 W/m²K. Roof U-value = 0.16 W/m²K, wall U-value = 0.18 W/m²K, floor U-value = 0.18 W/m²K. Same MVHR as House C.
  • House C 15 kWh/m²/yr (Passive House Institute limit for certification): airtightness 0.6 ACH (n₅₀ at 50 Pa). Triple glazing: Ug = 0.60 W/m²K, g-value = 0.50, Uf = 1.00 W/m²K, glazing-edge psi-value = 0.04 W/mK, window Uw-value = 0.8 W/m²K. Roof U-value = 0.13 W/m²K, wall U-value = 0.13 W/m²K, floor U-value = 0.11 W/m²K. MVHR.
• Hot water and other electricity: Hot-water use and non-space-heating electricity (such as lighting and appliances) are assumed to be the same across all three scenarios and are excluded. The comparison covers space heating only. House A uses extract ventilation while Houses B and C use MVHR. Differences in fan electricity, filters and equipment replacement are excluded unless the equipment option is ticked (see below).
• Fuel: Illustrative prices, including 5% VAT. Heat pump option: electricity at 33p/kWh with a seasonal coefficient of performance (SCOP) of 3.0. Oil option: kerosene at 115p/litre (prices vary with delivery size and supplier) with an 85% efficient boiler. The same heating system is applied to all three houses. No daily standing charge is included; many Northern Ireland electricity tariffs do not have one, and where a tariff does, it would be the same for all three houses and would not change the comparison. House C would not typically have an oil-fired boiler; the oil option is included for illustration and shows that the heat pump is generally the cheaper option. Heating costs rise at 3% a year over the 25 years.
• Heating CO₂e emissions: The calculator uses 0.196 kg CO₂e per kWh of purchased electricity and 0.247 kg CO₂e per kWh of heating oil (kerosene), from the UK Government (DESNZ) 2025 greenhouse gas conversion factors. The electricity figure includes grid transmission and distribution losses. Heat demand is divided by the heat pump’s SCOP or the boiler’s efficiency before applying these factors. The estimates cover space heating only. The electricity factor is a UK-wide average held constant for 25 years: Northern Ireland’s grid mix differs from the UK average, and the grid is expected to decarbonise over time, which would lower the heat pump figures further. Actual emissions depend on the electricity mix, fuel supplied and heating-system performance.
• Mortgage: 25-year capital-and-interest repayment on the build cost only — no site, fees or VAT — 100% borrowed for a like-for-like comparison. Houses A and B pay the SVR entered above; House C pays 1.5 percentage points below the SVR when the green discount is switched on. Only House C receives a discount in this selected comparison, and only when that option is enabled. Other homes may qualify for lender-specific discounts based on their energy rating, modelling, certification and the lender’s eligibility criteria. Any selected discount applies from the first modelled payment. The model does not simulate staged construction borrowing or the period before a completed property qualifies for a discount. Actual discounts may apply only after completion and submission of the required evidence. Mortgage rates are held constant throughout the modelled period. Actual rates, eligibility, fees and product terms may change. These figures are illustrative and are not a mortgage offer.
• Mortgage and project-cost basis: The calculator assumes borrowing equal to 100% of the stated construction cost. It excludes land purchase and project-specific costs such as groundworks, utility connections, professional fees, external works and contingency. These additional costs vary widely, and they can typically add around 15–25% of the construction cost on some projects (a project-specific cost plan would confirm this figure), before land. The resulting percentage is an illustrative loan-to-cost comparison, not a loan-to-value (LTV) figure; actual project costs, lender valuations and mortgage terms will vary.
• Build costs: a single £/m² rate is entered and applied identically to all three houses, so build cost and mortgage principal are the same for A, B and C unless a fabric premium is switched on. Excludes site, fees and VAT. Qualifying new self-build work can be zero-rated for VAT.
• Fabric premiums (optional): the two checkboxes add 2.5% to House B’s £/m² rate and 5% to House C’s £/m² rate, to test the comparison when the higher standards are priced at a premium rather than at parity. Both are off by default; the uplifts are illustrative placeholders pending a QS cost plan. Any premium should be read as a net figure: House C’s much smaller peak heat load (2.2 kW against 6.0 kW for House A at 200 m²) allows a smaller heat pump and fewer or smaller emitters, which can offset part of the extra fabric cost.
• Ventilation, equipment and certification (optional): When selected, the model adds: House C certification and extra airtightness testing (£5,000, paid upfront in cash at the start, not borrowed); fan electricity of 100 kWh/yr for House A’s extract fans and 350 kWh/yr for the MVHR in Houses B and C, at the same electricity price; MVHR filters at £100/yr for Houses B and C; replacement of the heat pump or boiler in year 18 (2044), costed at £3,000 plus £700 per kW of peak heat load, so a smaller system costs less; and replacement of the ventilation equipment in year 20 (2046) at £2,500 for MVHR or £600 for extract fans. Running costs rise at the fuel inflation rate and replacement costs are entered in today’s prices and inflated at 3% a year. With the heating-bill overpayment option ticked, ventilation running costs are included in the shared monthly budget alongside heating. These are illustrative allowances, not quotes; servicing and other maintenance are not included.
• Peak heat load: calculated in PHPP as 30 / 24 / 11 W/m² for Houses A, B and C.
• Floor-area basis: The calculator applies the entered gross internal area (GIA), rather than the Passive House treated floor area (TFA). TFA is typically 10% smaller for a standard-sized house, so applying the same heating-demand rate to GIA gives a higher modelled space-heating total than using TFA. This is an area-basis assumption, not a separate allowance for weather or occupant behaviour; actual energy use will vary with climate, occupancy and how the home is operated. Changing area scales the fixed demand rates; it does not rerun the PHPP calculations for a redesigned building.
• Overpayment option: Every house is assumed to budget the same monthly outgoing. With the heating-bill option ticked, this is a shared budget for mortgage and heating together: the highest of the three houses’ scheduled repayment plus heating bill, rising each year as fuel prices rise. Each house pays its own heating bill from that budget and puts the rest into its mortgage, so lower repayments and lower heating bills both become overpayments. With the heating-bill option unticked, the budget is the highest of the three scheduled repayments and only the difference in repayments is overpaid. Overpayments compound against the outstanding balance and clear the loan early. Once the balance reaches zero the mortgage payments stop, and the freed monthly cash is ignored unless the reinvestment box is also ticked.
• Overpayment limits: The calculator assumes that the modelled overpayments can be made without penalty. Actual allowances and early-repayment charges vary by lender and mortgage product. Check the chosen product’s terms before relying on these results.
• Solar PV and battery storage: These are not included, to keep the calculator simple and focused on comparing space-heating demand and mortgage costs. Their costs and savings depend on system design, when electricity is generated and used, and export payments.
• Reinvestment (optional): When selected, the monthly amount each house was paying into its mortgage continues to be paid into savings after the mortgage clears (with the heating-bill option ticked, this is the shared budget less that house’s own heating bill, so it rises over time), at the entered annual rate compounded monthly. The net figure includes those savings contributions as cash paid, then subtracts the accumulated savings pot. The full savings pot is shown as an asset accumulated after mortgage payoff. In the net comparison, payments into savings are included before the pot is deducted, so the same freed monthly payments are not counted twice. Savings returns are illustrative and not guaranteed. Figures are nominal and gross of tax, with no allowance for tax on interest, ISA limits or inflation adjustment.
• Model calendar: The model uses a fixed illustrative starting year of 2026. Year 18 corresponds to 2044 and year 20 to 2046, when any selected heating and ventilation replacements are applied. Year 24 corresponds to 2050, when any selected retrofit allowance is applied. Year 25 corresponds to 2051, when the comparison ends and any selected resale premium is applied. These dates do not move automatically with the current calendar year.
• 2050 retrofit (optional): When selected, the model includes an illustrative retrofit allowance in year 24 (2050). House A receives the full entered retrofit cost per m² multiplied by the floor area; House B receives 60% of that amount. These proportions are scenario assumptions, not assessments of work that either house will necessarily require. No retrofit allowance is included for House C in this scenario. The allowance is entered in today’s prices and inflated at 3% a year to 2050 prices, to match the nominal basis used for fuel costs. It is applied as a single lump sum, with no modelled reduction in heating costs afterwards. It is not a prediction of future legislation or required upgrades. Certification does not establish that a home will never need maintenance, replacement equipment or future improvements, nor does it guarantee a resale premium.
• Resale premium (optional): When selected, House C is credited an assumed resale premium at year 25, calculated by applying the entered percentage to its modelled construction cost — not its market value. This explores a possible additional sale value associated with independently certified building performance. It is a credit against the modelled total and would only be realised on sale. Certification does not guarantee a resale premium. Treat the default 15% as an illustrative placeholder pending local sales evidence. If the 2050 retrofit option is also selected, Houses A and B are assumed to have been upgraded a year before the resale premium is applied. A buyer might then see less difference between the houses, so combining both options may overstate House C’s advantage. The calculator shows a caution and the result without the premium when both are selected.
• Money is nominal: both charts show cash as paid, with no discount rate or net-present-value adjustment applied. Fuel is inflated at 3%/yr; the mortgage rate is held flat.
• What the total measures: with overpayment off, the total is 25 years of mortgage plus 25 years of heating. With it on, it is all mortgage cash paid — over a shorter term — plus the same 25 years of heating. Both cases cover the same 25-year window, so the figures stay comparable.
• Home ownership assumptions: Maintenance, insurance, rates, land, deposit, site works and utility costs, other than modelled space-heating costs, are excluded.
• “Passivhaus principles” and similar claims: The UK Passivhaus Trust has published a paper that uses two case studies to demonstrate how adopting Passivhaus without clear performance and comfort criteria, accurate PHPP modelling, and rigorous quality assurance can undermine ambitions for lower operational carbon emissions and energy bills, resulting in poor performance and significant costs. Some projects nevertheless claim to follow “Passivhaus principles” simply by incorporating selected building measures, while neglecting the integrated approach essential to achieving the standard’s intended outcomes. Download this paper.

General Disclaimer: This calculation tool is illustrative only. Mortgage products, rates and green discounts vary by lender and may be fixed or variable. This calculator illustrates a 1.5-percentage-point discount to the entered SVR; it is not a mortgage quote or confirmation of eligibility. Check current product terms, discount criteria and overpayment limits with the lender. Kelly Studio Architects is not regulated to give mortgage advice. Seek independent financial advice and check current rates with your lender or a regulated adviser. Kelly Studio Architects is not affiliated with or sponsored by any financial provider, lender or mortgage broker, and does not endorse any financial product.

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