Inter-war semis are among the most common homes in outer London, from the estates of Havering and Bexley to the suburban streets of Sutton and Barnet. This guide sets out a sensible order of work for them, how it fits with an extension or loft conversion, and the regulatory points to know before you start. It is general information, not a retrofit design service.
Why fabric first?
The usual advice is to reduce the heat the house loses before you change how you make heat. The Energy Saving Trust says an uninsulated home loses around 25% of its heat through the roof and about 33% through the walls. Those are national figures for uninsulated homes, so your own house will differ.
A practical order for an inter-war semi is:
- Loft insulation and draught-proofing.
- Walls: find out what you have, then decide.
- Floors and windows.
- Heating and ventilation, once the building has settled.
Step 1: loft insulation and draught-proofing
The loft is usually the simplest place to start. The Energy Saving Trust says topping up from around 120mm to 270mm, the minimum recommended for new builds, is worthwhile, and that a damp loft needs professional advice and the cause fixed first. It also says to keep a small ventilation gap between insulation and any boards to prevent condensation.
Draught-proofing comes with a warning. The Trust says good ventilation is still essential: do not block extractor fans, airbricks, wall vents or trickle vents, or fixed ventilation needed by flueless or open-flue gas appliances. If a home has none of these, it may already be too airtight for more draught-proofing.
Step 2: cavity or solid walls?
The Energy Saving Trust says houses built after the 1920s are probably cavity-walled, while earlier houses more often have solid walls. Brickwork with an even pattern of lengthways bricks suggests a cavity, and alternating end-on bricks suggest solid. A brick wall between 260mm and 350mm thick is likely to be a cavity wall. If walls are rendered or clad, or you are unsure, ask a building professional.
Cavity wall insulation may not suit homes that are exposed to extreme weather or flooding, in poor condition, at risk of overheating, or built with cavities under 50mm. Repair brickwork, roofs, gutters and downpipes before adding wall insulation, or you may trap damp.
Where walls turn out to be solid, internal or external wall insulation are the routes. External wall insulation changes the look of the house and is where planning most often comes in (see below).
Step 3: floors and windows
Gaps around skirting boards and floorboards are a common draught route, so the Trust lists them among places to check. Do not seal airbricks that keep timber floors dry.
For windows, Approved Document L sets the rule: a whole replacement unit should be draught-proofed and meet the stated standards, with insulated cavity closers where appropriate. Replacing glazing or a window in its existing frame does not need to meet them. If you enlarge a window or make a new opening, the total glazed area should not exceed 25% of the dwelling’s floor area, otherwise compensating measures are needed.
Step 4: heating and ventilation
Heating comes last in this order because a better-insulated house may need less from its system. Approved Document L says particular attention should be paid to Approved Documents F and J (ventilation and combustion appliances) when making an existing dwelling more airtight. Moisture is the main risk: tighter houses with poor ventilation can develop condensation and damp. Keep or improve extract fans in kitchens and bathrooms, and treat ventilation as part of the design rather than an afterthought.
When an extension or loft conversion is happening anyway
If you are already extending or converting the loft, the building is open and the fabric work can be done once. When a dwelling is extended, Approved Document L says new thermal elements and windows must meet the standards for new work, and existing fabric that becomes a thermal element must meet the limiting standards using the renovation guidance. A loft conversion that turns an unheated space into heated living space is a change to energy status, so existing thermal elements and new or replaced elements must meet the relevant standards.
The Building Regulations package for an extension or loft shows the insulation build-ups and junction details the builder works to. We prepare that package for extensions, lofts and alterations. See building regulations drawings, house extensions and loft conversions.
When Part L applies to renovation work
Approved Document L, Volume 1 (Dwellings) lists what counts as renovating a thermal element: new cladding or render on the outside, dry-lining inside, stripping an element back to the structure and rebuilding, replacing a flat-roof waterproof membrane, and providing cavity wall insulation. If more than 50% of an individual element’s surface is renovated, or more than 25% of the building envelope’s surface area (a major renovation), the whole element should be improved to the stated U-value. Regulation 23 applies this so far as is technically, functionally and economically feasible.
A 2026 edition of Approved Document L now exists alongside the 2021 edition incorporating 2023 amendments. Which applies depends on your project, so check with building control.
Planning permission, conservation areas and solar
- External wall insulation or cladding. Planning permission may be required if it affects the external appearance. Similar-looking materials are likely not to need it. In a conservation area you must apply first, and listed buildings need listed building consent.
- Article 4 directions. These are local rules that remove permitted development rights, so check your council’s map. Our conservation areas guide explains the wider picture.
- Solar panels. Roof panels are normally permitted development within the limits above, with extra restrictions in conservation areas.
- Energy efficiency in conservation areas. Part L allows some flexibility where full compliance would unacceptably alter character or appearance.
If you want certainty that work is lawful, a Lawful Development Certificate records it.
Sources
- Approved Document L, Volume 1: Dwellings (2021 edition incorporating 2023 amendments), GOV.UK
- Conservation of fuel and power: Approved Document L, GOV.UK
- Do I need planning permission for external insulation? Planning Portal
- Solar equipment mounted on a house, Planning Portal
- Cavity wall insulation, Energy Saving Trust
- Solid wall insulation, Energy Saving Trust
- Roof and loft insulation, Energy Saving Trust
- Draught proofing, Energy Saving Trust
