Orangery vs Extension vs Conservatory: Which Is Right for Your Home?

**Conservatory** (traditional definition): A structure attached to a dwelling where more than 50% of the roof area and more than 75% of the wall area is glazed — using translucent or transparent materials (polycarbonate or glass). The critical regulatory consequence of meeting this definition is the conservatory exemption under Part L (thermal) and Part O (overheating) of the Building Regulations: a conservatory meeting the area thresholds above is exempt from the energy efficiency requirements of Part L, provided it is thermally separated from the dwelling by an insulated wall and door (and the heating is controlled independently).

A conservatory that does NOT meet the thermal separation condition — or that does not meet the area thresholds — is subject to full Part L compliance.

*Traditional conservatories: polycarbonate or glass roof on a uPVC or aluminium frame*. The design is recognisably a conservatory — translucent roof, large glazed walls, minimal masonry or solid walling. Energy performance in cold weather is poor; overheating in warm weather is almost universal unless high-performance glass is specified.

*Thermal performance (polycarbonate conservatory)*: A 16mm twinwall polycarbonate panel has a U-value of approximately 1.9–2.5 W/m²K — significantly worse than a modern wall or even a double-glazed window. In cold conditions, the space is unusable without significant heating; in warm conditions, it becomes a greenhouse. For a year-round room the performance of a standard conservatory is fundamentally inadequate.

• **Orangery** (the term now most commonly used):
• Originally, an orangery was a formal greenhouse used for overwintering citrus trees — a solid masonry structure with large south-facing windows. In contemporary construction, the term 'orangery' is used to describe a structure with:
• •Predominantly solid masonry or timber/aluminium walls (not primarily glazed walls)
• •A central roof lantern or atrium rooflight (a glazed section in an otherwise solid flat or pitched roof)
• •A more permanent, room-like quality than a traditional conservatory

An orangery typically does NOT meet the conservatory exemption (because the wall area or roof area thresholds for the exemption are not met) — and is therefore subject to full Part L thermal compliance. This means it must be built to the same insulation and energy performance standards as a conventional extension.

From a planning perspective, an orangery is treated as an extension — not a conservatory — because it does not meet the area thresholds. All the same planning permission rules apply.

**Extension**: Any addition to a building in which the structural elements (walls, roof, floor) are built to Building Regulations-compliant standards in solid construction. An extension may include large glazed areas (bifold doors, rooflights, structural glass walls) — but the structure itself is solid, insulated, and thermally compliant. An extension with a partly glazed roof (rooflights making up less than 50% of the roof area) is an extension, not a conservatory.

Extensions are always subject to full Part L compliance — walls, roof, and floor must meet Part L1B U-value standards (wall 0.28 W/m²K, roof 0.18 W/m²K, floor 0.22 W/m²K as maximum U-values for new elements).

**Conservatories and planning permission**: Traditional conservatories can benefit from permitted development rights under Class A of Schedule 2 Part 1 of the GPDO — in the same way as any single-storey rear extension. The conservatory is NOT granted automatic planning permission just because it is a conservatory — the PD conditions (maximum depth, height, width, not on designated land) must still be met.

The common misconception is that conservatories are automatically PD. They are not — they are just treated the same as any other single-storey rear extension under PD rules.

**Orangeries and planning permission**: Orangeries are extensions — they are treated by the planning system in exactly the same way as a conventional brick-and-block extension. PD eligibility is determined by the same Class A conditions: depth, height, width, materials. If the orangery exceeds the PD thresholds, planning permission is required.

**Extensions and planning permission**: Same PD rules as above. The structural form (brick walls, solid roof, glazed elements) does not affect PD eligibility — only the dimensional and siting conditions matter.

**Conservation Areas**: In a Conservation Area, PD rights for extensions are removed or restricted. Planning permission is required for any enlargement of a dwelling that would be visible from a highway — including most rear extensions and conservatories visible from the side or rear access. The local planning authority's design guidance for Conservation Areas typically sets out requirements for the character and appearance of extensions — contemporary glass extensions and orangeries are sometimes permitted in Conservation Areas where the design is clearly distinguishable from the original building rather than attempting to replicate it.

**Thermal performance of the three types**:

• *Standard polycarbonate conservatory*:
• •Roof U-value: 1.9–2.5 W/m²K (well above Part L maximum of 0.18 W/m²K)
• •Wall U-value: typically 1.4–2.0 W/m²K (above Part L maximum of 0.28 W/m²K)
• •Overall: a cold, draughty space in winter; an overheating greenhouse in summer
• •Part L exempt only if thermally separated from the dwelling
• •Verdict: appropriate only for occasional use as a garden room or lightweight sun-trap — not for year-round habitation or as a kitchen extension

• *Self-cleaning or solar-control glass conservatory*:
• •Roof U-value: 0.9–1.4 W/m²K with advanced double-glazed units
• •Still above Part L requirements
• •Solar control glass reduces overheating in summer but remains thermally poor in winter
• •Verdict: an improvement on polycarbonate but still fundamentally unsuitable for year-round use in a UK climate without supplementary heating

• *Orangery (solid walls, glass lantern roof)*:
• •Wall U-value: 0.18–0.28 W/m²K (solid insulated masonry or SIPs wall — Part L compliant)
• •Lantern roof U-value: 0.9–1.4 W/m²K (double or triple-glazed structural glass lantern)
• •Solid flat roof sections: 0.18 W/m²K (100mm PIR, warm flat roof)
• •Overall: significantly better than a conservatory; cold-bridge-free at wall junctions with careful detailing; the lantern roof is the weakest thermal element
• •Verdict: a genuinely habitable year-round space; performs similarly to a conventional extension in energy terms; the overhead glazing creates a distinctive light quality that cannot be replicated by a conventional extension with only rooflights

• *Extension (solid construction, rooflights as required)*:
• •All elements Part L compliant: wall 0.18–0.28 W/m²K, roof 0.16–0.18 W/m²K, floor 0.22 W/m²K
• •Rooflights (up to Part L maximum 25% of floor area) 1.0–1.6 W/m²K (double-glazed) or 0.7–1.0 W/m²K (triple-glazed)
• •Overall: best thermal performance of the three types; year-round habitable; most similar to a heated interior room in thermal character
• •Verdict: the highest-performing option in energy terms; the design flexibility is greatest (brick, render, timber cladding, SIPs, zinc); the light quality depends entirely on glazing strategy

**2025 London costs**:

*Polycarbonate/glass conservatory (uPVC or aluminium frame, installed on existing patio or simple foundation)*: £15,000–£40,000 depending on size and glazing specification. The cheapest option — but the worst long-term performing.

*Aluminium frame glass-roof conservatory (hardwood or aluminium, proper strip foundation, concrete floor, double-glazed panels)*: £35,000–£70,000 — a better quality version that still has the thermal limitations of the conservatory form.

*Orangery (solid masonry or SIPs walls, central glass lantern, Part L compliant, on strip foundations)*: £60,000–£110,000 for a typical 12–20m² orangery shell with central lantern, bifold doors, and kitchen-connection opening; kitchen fit-out is client-supplied and additional.

*Brick/block extension with rooflights (comparable size to orangery)*: £55,000–£95,000 for a 12–20m² extension in shell form. The extension and orangery are comparable in cost — the choice is one of design character rather than budget.

*All second-fix materials (kitchen units, tiles, light fittings, flooring) are client-supplied under RCB standard terms.*