SAP Calculations for Conversions & Change of Use
Frequently Asked Questions

A SAP calculation for a conversion is the energy assessment that shows a building converted into a new home meets Part L of the Building Regulations, and produces the EPC for that new home.

You'll also see it called a "change of use SAP", a "conversion SAP" or an "L1B SAP" (after the older name for the part of Approved Document L that covered existing buildings). They all mean the same thing: a SAP (Standard Assessment Procedure) calculation for a dwelling created from a building that wasn't previously a home, or wasn't previously that home.

It uses the same SAP methodology and software as a new build assessment, but the target it's measured against is different, which is covered further down this page. If you'd like the basics of what SAP is and what the results mean first, our new build SAP FAQ covers that in more detail.

Its immediate purpose is practical: without it, building control can't sign off the conversion, and the new home can't be given an EPC.

Yes, if your project creates a new dwelling through a "material change of use", you'll need a SAP calculation and an EPC before building control can sign it off.

Material change of use is defined in Regulation 5 of the Building Regulations 2010. In practice, the most common examples we see are:

• Barn and agricultural building conversions
• Office, shop or other commercial buildings converted into flats or houses
• A house split into two or more flats
• Any building that ends up containing more dwellings than it did before

Because a brand new dwelling is being created, it needs its own EPC, and producing one requires a full SAP calculation. So in England and Wales, every conversion that creates a new dwelling needs a SAP calculation. Scotland handles conversions differently, which is covered further down this page.

If you're not sure whether your project counts as a conversion, get in touch and we'll confirm what you actually need before you commit to anything.

Not usually. Despite the name, a loft, garage or conservatory conversion isn't normally a "conversion" in the building regulations sense, because it doesn't create a new dwelling.

Turning part of your existing home into extra habitable space is treated as an extension or alteration to that home, not a change of use. These projects are normally shown to comply with Part L in one of two ways:

• The area weighted U-value method, which is usually the simplest and cheapest option where it works.
• The whole dwelling method, a SAP-based calculation of the whole house, often used where the design has more glazing than the simpler method allows.

The exception is where the work creates a genuinely separate, self-contained dwelling, for example a garage converted into a flat that will be lived in independently of the main house. That would count as a conversion, and would need a full SAP calculation and EPC.

If you're not sure which applies, get in touch. We'll point you to whichever option is genuinely the most cost-effective for your project, and won't charge you for a more detailed assessment than you actually need.

The calculation itself works the same way, but a conversion isn't expected to perform like a brand new house.

A new build SAP compares your design against a "notional" new build home. A conversion SAP instead compares it against a notional version of your conversion, where every item is set to the minimum standard that Approved Document L accepts for a conversion.

Importantly, those minimum standards for walls, floors, roofs, windows and doors differ depending on whether each element is:

• Existing and being kept as it is
• Existing and upgraded, for example an old wall being insulated
• New, such as a new extension wall or replacement windows

So an old solid stone wall isn't held to the same standard as a brand new cavity wall, and weaker existing elements are accounted for fairly. In practice, that means:

• Existing walls, floors and roofs don't all need to reach new build levels of insulation. Areas that are difficult or expensive to upgrade can often be balanced out by better performance elsewhere, such as the heating system, windows or insulation in the areas that are easier to improve.
• Large amounts of glazing aren't ruled out. The notional version assumes windows and doors make up 25% of the floor area, and your actual glazing is assessed against that. So a heavily glazed barn conversion can still comply, as long as the extra heat loss is made up elsewhere.
• Parts of the existing building can be assessed using recognised default values where the exact construction isn't known (see the next question).
• An air tightness test isn't usually required, unlike most new builds.

This flexibility is where an experienced assessor adds real value, because there's usually more than one way to get a conversion over the line, and some are much cheaper than others.

That's very common with conversions, and it doesn't stop the assessment going ahead.

We gather as much information as we can from you first: the age of the building, what it's made of, any original drawings or surveys, photos, and details of any work done to it over the years. We then assign a heat loss value (U-value) to each existing element by cross-referencing what we know against the standard default values set out in Appendix S of the SAP methodology, which gives recognised figures for different construction types and building ages.

Default values tend to be on the cautious side, so the more reliable information you can give us, the more accurately your building can be represented, which sometimes makes the difference between a pass and a fail.

Any new or upgraded elements, such as new insulation you're adding to an existing wall, are assessed based on your actual proposed specification.

It's rarely the end of the road. Most conversions that fail at first can be brought into compliance with a few targeted changes to the specification.

We don't just tell you "pass" or "fail". We'll show you where the building is falling short and help you find the most cost-effective way to fix it, whether that's extra insulation in an area that's easy to access, a different heating system, or better-performing windows. Two minor design revisions are included with our conversion SAP assessments as standard, which is usually plenty to turn a fail into a pass.

This is also why it pays to get the SAP done at design stage. Changing a specification on paper costs very little; changing it once the work is done can be expensive and disruptive.

If your conversion is already failing, or you're worried it might, get in touch and we'll talk it through with you.

Not usually. Unlike most new builds, an air tightness (air pressure) test isn't normally necessary for a conversion.

Where no test is carried out, the SAP calculation uses a standard assumed value for how much air leaks through the building instead.

There is one situation where a test can be worth doing: if the converted building is well sealed and the SAP needs a small improvement to pass, a good test result can sometimes be what tips it over the line. It's not something we'd suggest by default, and we'll only recommend it if it's genuinely likely to be the most cost-effective option for your project.

Yes, it still needs a SAP calculation and an EPC, but Part L allows more flexibility where upgrading the building would harm its character.

Approved Document L specifically recognises that listed buildings, buildings in conservation areas and other traditional or historic buildings may not be able to take the same upgrades as a modern building, for example where external insulation or replacement windows wouldn't be acceptable, or where the wrong type of insulation could cause damp problems in older solid walls. In these cases, the aim is to improve energy efficiency as far as is reasonably practical without causing harm.

In practice, this is where the flexibility of the SAP calculation really helps: if the walls or windows can't be changed much, the difference can often be made up elsewhere, such as the heating system or roof insulation. It's worth involving your building control body and conservation officer early so everyone agrees what's acceptable.

We're happy to work alongside your architect and conservation officer to find a specification that works for everyone. Get in touch to discuss your project.

Yes, in England and Wales. Once the conversion is complete, the final "as built" SAP calculation is used to produce the new home's EPC (Energy Performance Certificate), which we lodge on the national register.

The process normally happens in two stages: a design stage SAP calculation before work starts, which shows building control that the design will comply, and an as built SAP calculation at the end, updated for anything that changed during construction. The EPC is produced from the as built calculation.

The EPC matters beyond building control sign-off. You'll need it to sell or let the property, and in England and Wales, rented homes currently need to achieve a minimum EPC rating of E.

Not under the Building Regulations at the moment. In England, Part O (overheating) currently applies only to new build homes, not to conversions.

However, it's quite common for a local planning authority to add a planning condition requiring an overheating assessment for a conversion anyway, especially for larger schemes such as office to residential conversions. So it's worth checking your planning decision notice for anything along those lines.

The rules may also change. The government has announced a review of Part O that includes looking at extending it to conversions, although no date has been set yet.

If your planning conditions do require an overheating assessment, we can carry out either a simplified overheating assessment or a full TM59 dynamic overheating assessment, whichever the condition actually calls for. Doing it alongside your SAP also means the two assessments can be kept consistent with each other.

Yes. England and Wales work in a very similar way, but Scotland treats conversions quite differently. We work on conversions across all three countries.

• England: Part L of the Building Regulations 2010, with guidance in Approved Document L Volume 1. Everything on this page is based on the English rules unless stated otherwise.
• Wales: Wales has its own version of Part L and its own Approved Document L, published by the Welsh Government. The overall approach to conversions is much the same as in England, including the SAP calculation and EPC, but some of the minimum standards differ.
• Scotland: Energy standards are set out in Section 6 (Energy) of the Scottish Building Standards Domestic Technical Handbook. For conversions, the main energy standard that calls for a SAP-based calculation (Standard 6.1) doesn't apply, and neither does the requirement to produce an EPC on completion (Standard 6.9). Instead, the converted building needs to meet the handbook's insulation and building services standards as far as is reasonably practicable, with the U-value targets depending on whether the building was previously heated or unheated.

So in Scotland, a conversion doesn't usually need a full SAP calculation to get its building warrant, although an EPC will still be needed later if the property is sold or let. It's important your assessor works to the right set of rules for where your project is. Wherever you're converting a building in Great Britain, get in touch and we'll confirm exactly what your project needs, and won't charge you for more than that.

Ideally at design stage, before work starts, and we'll need drawings of the building plus some details about its existing and proposed construction.

In general, we'll need:

• Scaled drawings of the building as it will be once converted: floor plans, elevations, sections and a site plan.
• Whatever you know about the existing building: its approximate age, what the walls, floors and roof are made of, and any surveys, original drawings or photos.
• Details of any new or upgraded insulation, windows and doors.
• The proposed heating, hot water, ventilation and lighting, plus any renewables such as solar panels or a heat pump.

When you accept a quote, you'll receive a full, detailed checklist of everything needed for your specific project.

Getting the SAP done at design stage costs no more than leaving it until the end, but it means any problems can be spotted and fixed on paper rather than on site. Price depends on the size and complexity of the conversion. Get an instant quote, or contact us for a no-pressure chat about your project.