Conversions are different
Creating a dwelling from an existing building — a barn, an office, a house divided into flats — is a material change of use under the Building Regulations, and regulation 6 applies Part L to it in every case. But the machinery is different from a new build. The target emission rate, the primary energy target and the fabric energy efficiency target all apply where a building is erected; a conversion is not erected, so there is no notional dwelling to beat and no design-stage and as-built notices under regulation 27.
Instead, compliance is shown element by element: the new parts have to meet one set of standards, the retained parts another, and where the retained fabric cannot reasonably get there, Approved Document L has a payback test. We assess both — how the existing walls, floors and roof can realistically be upgraded, and how the new heating, glazing and insulation are specified — and put it together in a calculation Building Control can follow.
The standards, in numbers
New thermal elements — anything you build new in the conversion — must meet the limiting values for new elements in existing dwellings: walls 0.18, floors 0.18, roofs 0.15, windows and glazed doors 1.4, other doors 1.4, rooflights 2.2 W/m²K.
Retained thermal elements are checked against a threshold and, if worse, must be improved:
| Retained element | Threshold — upgrade if worse than | Improve to |
|---|---|---|
| Roof | 0.35 | 0.16 |
| Cavity wall (cavity insulation) | 0.70 | 0.55 |
| Solid wall (internal or external insulation) | 0.70 | 0.30 |
| Floor | 0.70 | 0.25 |
Where the improved value is not technically or functionally feasible, or would not achieve a simple payback of 15 years or less, the element is improved to the best value that is feasible within a 15-year payback. That clause is what makes a solid-stone barn convertible; it is also a reasoned position, not an exemption, and the calculation needs to show the reasoning.
If you want to see where a particular wall, roof or floor lands against that table before you speak to anyone, RenoMonkey works the threshold and the 15-year payback test for any element, free and on screen, and the renovation guide explains how the tests fit together.
Windows and doors in an existing dwelling are a trap: each one has to meet 1.4 on its own, because they are excluded from area-weighting. Where a building’s character genuinely requires the original windows, the Approved Document accepts a centre-pane U-value of 1.2 or better, or single glazing with low-emissivity secondary glazing behind it.
Why a SAP, if there is no target
Because it is the only practical way to show the trade-offs. A converted building almost never reaches every improved value — the stone wall can take 60 mm of internal insulation, not 150; the floor is a slab you are not lifting — and what Building Control needs to see is that the dwelling as a whole is no worse than it would be with the elements brought to standard. A SAP models the actual building against that, lets us show where a better roof compensates for a wall that cannot be reached, and produces the figures the EPC is lodged from where one is required. Building Control bodies routinely ask for it on conversions for exactly that reason.
Two stages
Design — from your existing and proposed plans plus a specification, we check the proposal meets the requirements before work starts and tell you where it does not. No site visit or survey.
As-built — on completion we issue the final calculation and, where the regulations require it, the on-construction EPC for each new dwelling. The Energy Performance of Buildings Regulations require that EPC within five days of completion where a building is modified to have a different number of separately used parts and the fixed services are provided or extended — the flat split, the office block. A single building converted to a single dwelling is a grey area on that duty, but needs an EPC the moment it is sold or let regardless. We tell you which applies before you pay for either.
Common conversion problems
Solid walls and the internal-insulation condensation risk that comes with them — our condensation risk calculator screens a build-up to BS EN ISO 13788 free, which is usually enough to tell you early whether a lining is going to be a problem. Suspended timber floors over a void nobody has seen. A roof that is being retained with the insulation it has. Heritage windows. A heating system being extended into a part of the building that was never heated, which brings its own change-of-energy-status duty. None of these is unusual, and all of them are cheaper to resolve on the drawings than after the first inspection.
Related services
Converting and extending at the same time? See SAP for extensions. Planning conditions asking for renewables? See energy statements. On completion, sound testing is required where the conversion creates attached dwellings; an air test is not required on a conversion, but a measured result beats the default the calculation otherwise has to assume, and both can be bundled. Get a quote or call 01202 623236.
Common questions
Does Part L apply to a conversion?
Yes, in full. Creating a dwelling where there was not one before — or changing the number of dwellings in a building — is a material change of use under the Building Regulations, and regulation 6 lists Part L among the requirements that apply in every case. What differs from a new build is how compliance is shown.
Do I need a SAP calculation for a barn conversion?
Usually, in practice. A conversion has no target emission rate to beat, because that test only applies when a building is erected. Instead each element has to meet its own standard: new walls, floors, roofs, windows and doors to the limiting values for new elements in existing dwellings, and retained elements upgraded where they fall below a threshold. Building Control commonly asks for a SAP to demonstrate that, and it is the only sensible way to show the trade-offs when a retained stone wall cannot reach the improved value.
What U-values does a conversion have to achieve?
New elements: walls 0.18, floors 0.18, roofs 0.15, windows and glazed doors 1.4, other doors 1.4, rooflights 2.2 W/m²K. Retained elements that are worse than a threshold must be improved — a roof worse than 0.35 to 0.16, a cavity wall worse than 0.70 to 0.55, a solid wall worse than 0.70 to 0.30 with internal or external insulation, a floor worse than 0.70 to 0.25 — where that is technically and functionally feasible and pays back within 15 years.
What is the 15-year payback rule?
Where bringing a retained element up to the improved value is not technically or functionally feasible, or would not achieve a simple payback of 15 years or less, Approved Document L allows it to be improved to the best standard that is feasible within a 15-year payback instead. It is what makes a solid-stone barn convertible at all. It is not a blanket exemption — the reasoning has to be shown.
Can I keep the original windows in a conversion?
Only where the character of the building requires it. In an existing dwelling every window and door has to meet 1.4 W/m²K on its own — they are excluded from area-weighting, so a poor one cannot be averaged against a good one. Where that would harm the building's character, the Approved Document accepts a centre-pane U-value of no more than 1.2, or single glazing supplemented with low-emissivity secondary glazing.
Does a conversion need an EPC?
It depends on what the conversion does. The Energy Performance of Buildings Regulations require an on-construction EPC within five days of completion where a building is modified to have a different number of separately used parts and fixed heating, hot water, ventilation or air-conditioning services are provided or extended — a house split into flats, an office into apartments. A single building converted into a single dwelling sits in a grey area on that duty, but an EPC is needed the moment it is sold or let in any case. We tell you which applies.
Do I need design-stage and as-built notices for a conversion?
No — the design-stage and as-built notification duties in regulation 27 apply where a building is erected, not converted. But Building Control will still want to see the compliance evidence before work starts and confirmation of what was done at completion, and the practical process looks the same: a design calculation, then an as-built one.