Adding light to a Tonbridge extension has two main options: a Velux-style roof window set into the slope of a pitched roof, or a roof lantern — a raised, ridged glazed structure sitting on a flat roof. They serve different architectural situations, have very different costs, and have different planning implications. Here is the decision framework.
The Core Difference
A Velux window (or equivalent roof window) sits flush in the slope of a pitched roof, tilting on a centre or top pivot to ventilate. It requires an existing or new pitched roof surface to install into. Cost: £450–£750 per unit supply and fit.
A roof lantern is a self-supporting glazed structure — essentially a raised box of glass and aluminium — that sits on a curb (upstand) built onto a flat roof. It admits light through vertical and angled glazed faces and can be opened for ventilation. Cost: £2,500–£8,000 supply and fit depending on size and specification.
Thermal performance: Building Regulations Part L1B (Conservation of Fuel and Power in Existing Dwellings) requires that any new roof window installed as a replacement must achieve a maximum U-value of 1.6 W/m²K. Current Velux GGL and GGU units with double glazing achieve 1.3–1.4 W/m²K, satisfying this requirement. Source: Building Regulations Approved Document L1B, 2022
When Each is Appropriate
| Situation | Velux | Roof Lantern |
|---|---|---|
| Extension has a pitched roof | ✓ Ideal | ✗ Not applicable |
| Extension has a flat roof | ✗ Not applicable | ✓ Ideal |
| Loft conversion | ✓ Standard specification | ✗ Not applicable |
| Single-storey rear extension | Possible if pitched | ✓ If flat roof |
| Orangery / garden room | Possible | ✓ Characteristic feature |
| Low budget, maximum light | ✓ Better value | ✗ Higher cost |
| Statement architectural feature | Functional | ✓ Better aesthetics |
The decision is largely made by the roof form. If you have a pitched roof, Velux is the solution. If you have a flat roof extension, a roof lantern is the solution. The question only requires a genuine decision when you're designing a new extension and choosing the roof form.
Cost Comparison for a Tonbridge Kitchen Extension
| Option | Typical Cost | What's Included |
|---|---|---|
| 2x Velux GGL 780x980mm (pitched roof) | £900–£1,400 | Supply, fit, flashing kits |
| 1x Roof lantern 1000x1500mm (flat roof) | £2,500–£4,000 | Supply, fit, kerb, GRP upstand |
| 1x Roof lantern 1500x2500mm (flat roof) | £4,000–£8,000 | Supply, fit, kerb, GRP upstand |
| Flat roof conversion to pitched + 2x Velux | £8,000–£15,000 | Structural conversion |
Free survey and quote for Velux or roof lantern installation — call 0173 224 1138
📞 0173 224 1138Planning Considerations in Tonbridge
Velux windows on permitted development: Under Permitted Development rights, roof windows can be installed without planning permission if they don't protrude more than 150mm above the existing roof plane and are not on the principal or side elevation facing a highway. Most rear-of-house Velux installations in Tonbridge are PD-compliant.
Roof lanterns on permitted development: A roof lantern on a flat-roof extension is generally permitted development if the extension itself is PD-compliant and the lantern doesn't exceed 4 metres total height. In Tonbridge conservation areas, additional restrictions apply — we advise on this at survey.
Conservation areas and Article 4 directions: Some streets in Tonbridge town centre have Article 4 directions that remove certain PD rights. If your property is in a designated area, any roof alteration that would normally be PD may require a planning application.
Thermal and Condensation Performance
Both Velux windows and roof lanterns are available in double and triple glazing. For Tonbridge properties in the lower valley where humidity is above average, we recommend:
- Velux: GGU polyurethane-framed units rather than pine GGL for rooms prone to humidity (bathrooms, kitchens). The polyurethane frame handles sustained moisture without swelling.
- Roof lanterns: thermally broken aluminium frames are essential — non-broken frames create cold bridges that cause condensation on the inner face in Tonbridge winters.
- Warm edge glazing spacers in the sealed unit reduce condensation on the glass edges — worth specifying in lower-valley Tonbridge properties.