The four-condition building regulations exemption for conservatories
The building regulations exemption for conservatories is set out in Schedule 2, Class 7 of the Building Regulations 2010 (as amended). To qualify for exemption, all four conditions must be satisfied simultaneously. First, the conservatory must be at ground level — it cannot be a first-floor addition or an extension above an existing room. Second, the internal floor area must be less than 30m² — measured as the internal floor area of the conservatory only, not including any adjacent rooms. Third, the separation condition: the conservatory must be separated from the existing house by external-quality walls, doors or windows. This means the door between house and conservatory must be an external-quality door (solid or part-glazed, draught-stripped and fitted with appropriate hardware), not a standard interior door. Fourth, and most commonly misunderstood, the roof condition: the roof must be translucent or transparent in more than 75% of its area. This 75% threshold is precise. A conservatory with a flat opaque polycarbonate or solid GRP roof section that reduces the transparent area below 75% fails the exemption. A dual-pitched glazed roof with an aluminium ridge meets the condition. A lean-to with a glass roof meets it. An orangery with a solid insulated roof and a central lantern light almost always fails it — the solid roof area typically exceeds 25% of total roof area. Builderr calculates the transparent roof percentage for every project at design stage and confirms exemption status before specification is finalised.
What happens when the exemption fails — Part L, Part A and Part F
If any of the four exemption conditions is not met, the conservatory becomes a regulated extension and must comply with all relevant parts of the Building Regulations 2010. The most significant requirements are: Part L (Conservation of Fuel and Power): the 2021 edition (effective June 2022) requires any new extension to achieve a whole-building SAP (Standard Assessment Procedure) calculation, ensuring the extension does not worsen the existing dwelling's energy performance beyond the notional standard. In practice, this requires: minimum U-values for walls (0.18 W/m²K), floor (0.13 W/m²K) and roof (0.15 W/m²K); maximum U-values for windows and doors (1.4 W/m²K for the overall window, 1.2 W/m²K for the glazing unit itself); and overheating assessment under Part O. Part A (Structural): where the conservatory requires a new opening in the existing house wall, structural calculations for the lintel, padstones and load redistribution are required. An approved inspector or local authority building control officer must sign off on the structural design. Part F (Ventilation): regulated extensions require background ventilation (typically trickle vents in all windows) and provision for purge ventilation (openable windows meeting a minimum area). Part P (Electrics): any electrical installation in the conservatory must be notified to building control, either by a NICEIC or NAPIT-registered electrician (who can self-certify) or by submission to the local authority. Part O (Overheating): for new extensions completed after June 2022, Part O requires a simplified or detailed overheating risk assessment. Conservatories and orangeries facing south, west or southwest are particularly at risk — solar-control glazing with SHGC ≤0.35 and provision for natural ventilation are typically required to pass.
FENSA and CERTASS certification for glazing replacement
Even where a conservatory is exempt from building regulations under Schedule 2 Class 7, any replacement of existing glazed units or installation of new glazing must comply with building regulations Part L and Part K. In practice, this means all glazing must meet current energy performance standards (1.4 W/m²K maximum overall window U-value) and safety glazing requirements (toughened or laminated glass in critical locations — doors, low-level panels, roof lights). The compliance route for glazing is via a Competent Person Scheme — specifically FENSA (Fenestration Self-Assessment Scheme) or CERTASS, both of which are Government-authorised schemes. A FENSA or CERTASS-registered installer self-certifies compliance and issues a FENSA certificate, which is registered with the local authority and provided to you as a homeowner. This certificate is required on sale of the property — solicitors will ask for it. Builderr uses FENSA-registered glazing installers on every project and provides the certificate at handover. If a non-registered installer is used, the homeowner must notify the local authority building control directly and pay a regularisation fee (typically £100–£250), which adds delay and cost. The FENSA register is publicly searchable at fensa.org.uk.
Orangeries and solid-roof conservatories — full building regulations always apply
Orangeries — defined as structures with a solid insulated roof, brick or stone pillars, and a central lantern light — almost never qualify for the Schedule 2 Class 7 building regulations exemption. The solid roof, by definition, means the transparent roof area is less than 75% of the total. This means Part L, Part A, Part F and Part O all apply without exception. The SAP Part L calculation for an orangery requires: floor insulation to 0.13 W/m²K (typically 150mm PIR in a screed floor), wall insulation to 0.18 W/m²K (typically 100mm partial fill cavity with PIR), roof insulation to 0.15 W/m²K (typically 200mm rigid PIR between and below rafters), and glazing of 1.2 W/m²K centre-pane performance with low-E argon-filled units. The lantern roof element typically needs to achieve ≤1.4 W/m²K overall. Where the orangery connects to an existing house via a wide opening (over 1,800mm), Part A structural calculations for the lintel above the opening are always required. Structural engineers calculate the load from the first floor or roof above and specify the RSJ size, padstones and bearing length. Builderr coordinates the structural engineer, SAP assessor and building control inspector as part of every orangery project — these are not separate items clients need to arrange independently.
