How EPC Points Are Calculated: SAP and RdSAP 10 Explained

Key takeaway: an EPC score is the output of a government cost model, not a reading of your energy use. For an existing home it is a SAP rating from 1 to 100, band C starts at 69 points, and every improvement adds a broadly knowable number of points. Since 15 June 2025, assessments use RdSAP 10, an evidence-based methodology under which the paperwork you hand the assessor is worth real points.

Most owners meet their EPC as a letter on a certificate and never look at the number underneath it. That is a mistake, because the number is where every improvement decision starts: the gap between your current score and the next band threshold defines the whole project. This guide explains where the number comes from, what the assessor actually records, why two apparently identical homes can score fifteen points apart, and how commercial buildings are scored differently.

A model of cost, not a meter reading

For an existing home, the score is produced by RdSAP, the reduced-data version of the Standard Assessment Procedure (SAP) on GOV.UK. It is a standardised government model of what your home should cost to heat, light, ventilate and supply with hot water, per square metre of floor area, each year. The assessor surveys the fabric and the systems; the model computes a notional annual running cost; and a lower modelled cost per square metre produces a higher SAP rating.

Your actual bills never enter the calculation. Neither do your tariff, your thermostat habits, or how many people live there. That is why the certificate can feel wrong in both directions, a frugal household in a leaky solid-wall terrace still gets a low score, and a heavy-handed heating user in a well-insulated semi still gets a high one. The model is pricing the building, not the occupants.

Two consequences are worth fixing in mind before you spend anything. First, because it is a model, two similar homes carry different scores whenever the model was fed different inputs. Second, because the metric is cost per square metre, measures move the score in a fairly predictable order, which is what makes cost-per-point planning possible.

The bands: 69 is the number that matters

Domestic EPC bands run A to G on fixed SAP thresholds:

Band	SAP score
A	92+
B	81-91
C	69-80
D	55-68
E	39-54
F	21-38
G	1-20

The letter gets the attention; the number does the work. A mid-D home at 62 needs +7 points to reach C. A weak E at 40 needs +29, a different project on a different budget, even though both owners describe the task as “getting to a C”. Your current number is free to check on the government’s find an energy certificate service: pull the certificate, note the score, and subtract it from 69 (for C) or 81 (for B).

69 points, the threshold where band C begins. The gap between your current SAP score and 69 defines your improvement project in one subtraction.

What the assessor records, and what carries the most weight

The survey builds an inventory: wall construction and insulation, roof and loft insulation depth, floor type, window glazing, the main heating system and its controls, hot water arrangements, ventilation, fixed lighting, and any renewables such as solar PV or battery storage.

Because the model prices heat loss and running cost, the levers rank in a rough order. Walls come first, an uninsulated solid wall is the single biggest drag on most older certificates. Then the roof, then the heating system and its controls, then windows and draught losses, then hot water, then lighting, with renewables acting as a generation offset at the end.

That ranking is why the cheapest route up rarely starts where owners expect. Lighting and hot-water measures are small points but tiny money; glazing is visible money but small points. The full cost-per-point ranking, LED lamps and cylinder jackets before glazing, every time, is the core of our guide to raising your EPC score.

RdSAP 10: since 15 June 2025, paperwork is worth points

Domestic assessments lodged after 15 June 2025 use RdSAP 10, and it changed the job in four practical ways:

• Every window is measured individually. The old “typical window area” assumption is gone, so unusual glazing layouts are finally scored accurately, sometimes up, sometimes down.
• Heating efficiency is evidence-based. The assessor scores the actual boiler from its model number and manufacturer data. No documentation means pessimistic age-band defaults.
• Smart heating controls and battery storage are recorded. Kit that previously vanished from the certificate now counts.
• Surveys take longer and cost more. Expect up to around 90 minutes on site against the old 45-60, with typical fees up roughly 15-30%.

The net effect is blunt: undocumented improvements score as if they do not exist. A ten-year-old condensing boiler with no paperwork can be assessed on default assumptions that throw points away, and unevidenced loft insulation may be recorded at the depth the assessor can verify rather than the depth you paid for. Keep invoices, FENSA and MCS certificates, and insulation depth photos, we cover the full pack in our guide to EPC re-assessment after improvements.

Why two similar homes score differently

Two houses on the same street, built the same year, fifteen points apart. It happens constantly, and the reasons are usually mundane:

• Evidence. One owner showed the boiler manual and the cavity-wall guarantee; the identical boiler next door was scored on age-band defaults because nobody could find the paperwork.
• Heating fuel and system. The metric is cost-based, so electric room heaters model as expensive heat while a mains-gas condensing system with full controls models as cheap heat. Upgrading heating and controls is one of the strongest moves on most certificates for exactly this reason.
• Floor area. The score is calculated per square metre, so extensions and conversions change the arithmetic, the same heat loss spread across a different floor area models differently.
• The small stuff compounds. Halogen spots against LEDs, a bare hot-water cylinder against a jacketed one, a lone programmer against a programmer, room thermostat and TRVs. Each is worth a few points; together they explain most odd gaps.
• Windows are now measured. Under RdSAP 10, a home with a large glazed rear extension no longer scores as if it had typical window areas.

The remedy is the same in every case: know what the model was told about your home, evidence everything that exists, and then buy the missing points cheapest-first, starting with the fabric, via loft, cavity and floor insulation, where the survey shows it is needed.

Commercial buildings: SBEM in brief

Non-domestic buildings run on different arithmetic. Instead of a SAP score, a commercial EPC carries an asset rating modelled in SBEM (Level 3 for simple buildings, Level 4 for complex ones, with DSM, dynamic simulation modelling, at Level 5 for the most complex), where a standard benchmark building scores 100 and lower is better. The headline metric is the carbon-based Environmental Impact Rating, which the government confirmed it will keep for non-domestic EPCs in its partial response of 9 March 2026.

The improvement logic transfers, though: the cheap commercial points are almost always LED lighting with occupancy and daylight controls, then heating upgrades. The law today is a minimum of EPC E to let, including continuing lets since 1 April 2023, and the government’s interim response of 18 June 2026 proposes EPC B by 2031 for privately rented buildings over 1,000 square metres, a proposal subject to secondary legislation, not yet law.

The reform on the way: four metrics, targeted from October 2026

The scoring system itself is changing. In its partial response to the EPC reform consultation, published 9 March 2026, the government confirmed that domestic EPCs will move from a single cost metric to four headline metrics, energy cost, fabric performance, heating system and smart readiness, produced with the Home Energy Model in place of SAP and RdSAP. New-style domestic EPCs are targeted from October 2026, subject to the 2026 regulations, and industry reporting suggests the date could slip, so check gov.uk before relying on it. Ten-year validity is retained, and existing certificates remain valid for their full term.

For improvers, the strategy is reassuring rather than alarming: insulation feeds the fabric metric, efficient heating and controls feed the heating and smart metrics. Fabric-first, controls-second scores well under the current regime and the reformed one, whereas a rating propped up by one strong area may look different once the score is split four ways.

Turning the arithmetic into a plan

The planning sequence falls out of everything above. Find your current score on the register. Subtract it from the threshold you need. Then buy that many points in cost-per-point order, starting with the quick wins under £500 before anything with four figures on the invoice. Typical prices for every measure are on our guide to what each measure typically costs, and for a fully worked band jump, three published benchmarks plus a documented case, see what it really costs to go from EPC E to C.

Common questions

Is my EPC score based on my energy bills?

No. The score is a modelled cost of running the building per square metre, computed by RdSAP from the surveyed fabric, heating, hot water and lighting. Your bills, tariff and usage habits play no part, which is why changing supplier or cutting consumption never moves the certificate, and why fabric and system improvements always can.

Can I work out my own score before paying for an assessment?

Roughly, yes. Your existing certificate on the national register shows the current score and a recommendation report that estimates the rating change for each suggested measure, that is the government’s own arithmetic, free to read. Published points-per-measure ranges then let you sketch the gap. Treat all of it as planning data, confirmed for your property by a fresh assessment, and see common EPC score questions for the figures we get asked about most.

Know your number before you spend a pound

Everything in EPC improvement flows from one subtraction: current score minus target threshold. An accredited domestic energy assessor can confirm your inputs, correct what the model has been told, and price the cheapest route to the band you need, request a free quote and start from the number, not the letter.