Extensions and Conversions - L1b
Extensions
The building standards that make up the approved document L1b are as follows. These as with L1a are satisfied with an energy assessment using SAP 2009.
To avoid energy assessor involvement, extensions may be built to stand alone figures but must adhere to further strict guidelines. These are to have no more than 25% of the new floor area in m2 as openings. Heating systems and controls must also meet the standards set out on the latest building services compliance guide. Both Part L and The Building Services Complaince Guide can be download from our downloads section.
Limiting U values for conversions
| Element | SAP upper limit (according to appedix c) | Stand alone value |
| Wall | 0.7 | 0.28 |
| Roof - insulation at ceiling level | 0.35 | 0.16 |
| Roof - insulation at rafter level | 0.35 | 0.18 |
| Roof -flat | 0.35 | 0.18 |
| Floor | 0.7 | 0.22 |
| Swimming pool basin | - | 0.25 |
| Windows | - | 1.6 |
| Door >50% glazed | - | 1.8 |
| Door | - | 1.8 |
Another calculation method accepted by building control is the area weighted average method of calculation. This method has its merits, but it is unable to assess any upgrades made to the original building that are often the case with extensions e.g. new boilers or extra roof insulation.
Green House Calcs energy assessors are glad to provide this calculation if required, but recommend full calculations under SAP 2009. In this way upgrades to the existing building can be included, suggested as a cost effective solution if necessary and avoid any extra fees should thearea weighted calculations not provide the pass required by building control.
ConversionsBuilding conversion projects work in a very similar way to extensions, though as the elements of the building nearly always exist, and are often in a state of disrepair, the u-value target for upgrade is lower. The following tables note these values.
Limiting U values for conversions
| Element | SAP upper limit (according to appedix c) | Stand alone value |
| Wall | 0.7 | 0.3 |
| Roof - insulation at ceiling level | 0.35 | 0.16 |
| Roof - insulation at rafter level | 0.35 | 0.18 |
| Roof -flat | 0.35 | 0.18 |
| Floor | 0.7 | 0.25 |
| Windows | - | 1.6 |
| Door >50% glazed | - | 1.8 |
| Door | - | 1.8 |