Garage conversion · England · 20 build-ups · 36 notes
Building Regulations specification for a garage conversion
A garage conversion is mostly a job of bringing existing fabric up to the standard for a habitable room: the slab, the single-leaf walls, the roof, the door opening that becomes a wall. The specification treats each as the retained or new element it is, with the U-value it has to meet.
Assembled in Specline from a library written to the Approved Documents, with the U-value working for every calculated element printed as its own section, and issued under your practice's own name.
What building control looks for
The questions the specification answers before they are asked
On a garage conversion
- The infill to the garage door opening as a new wall, and its foundation.
- The floor over the existing slab, insulated to the new-element limit with the damp proof membrane linked to the walls.
- The existing walls and roof as retained thermal elements: whether they cross the Table 4.3 threshold and what they must be improved to.
- Separation from the rest of the dwelling where a garage remains, and the door between them.
- Ventilation and means of escape for the new room, and whether the change is exempt from any part of the Regulations.
The limits that apply — Approved Document L Volume 1
| New element (Table 4.2) | Maximum |
|---|---|
| Walls | 0.18 |
| Floors | 0.18 |
| Roofs | 0.15 |
| Windows | 1.4 (or WER band B) |
| Rooflights | 2.2 (or band C) |
| Doors | 1.4 (or DSER B/C) |
| Retained element (Table 4.3) | Threshold | Improve to |
|---|---|---|
| Roof, all types | 0.35 | 0.16 |
| Cavity wall | 0.70 | 0.55 |
| Other walls | 0.70 | 0.30 |
| Floor | 0.70 | 0.25 |
W/m²K. A new element meets Table 4.2. An existing element renovated as part of the work is improved where it is worse than the Table 4.3 threshold.
Part A · construction build-ups
The build-ups in the library for a garage conversion
Each is written layer by layer with named products and their alternatives, and each figure is what the build-up achieves when calculated to BS EN ISO 6946, or BS EN ISO 13370 for floors and basements, not the target it has to beat. Cavity walls, framed walls, floors, basements and roofs can also be configured layer by layer on the job.
| Group | Build-up | Achieves |
|---|---|---|
| Foundations | ||
| FD | Trench Fill Foundation | Wall + 300 wide, 1000 deep |
| FD | Strip Foundation | Wall + 300 × 225, 1000 deep |
| External walls | ||
| EW | Infill to Garage Door Opening — Cavity Wall | 0.17 W/m²Klimit 0.18 |
| EW | Infill to Garage Door Opening — Timber Frame with Brick Outer Leaf | 0.16 W/m²Klimit 0.18 |
| EW | Infill to Garage Door Opening — Solid Masonry with Internal Insulation | 0.17 W/m²Klimit 0.18 |
| EW | Existing Single-Leaf Garage Wall — New Inner Leaf Forming a Cavity Wall | 0.24 W/m²Klimit 0.30 |
| EW | Existing Uninsulated Cavity Wall — Blown Cavity Insulation | 0.28 W/m²Klimit 0.30 |
| EW | Existing Garage Wall — External Wall Insulation | 0.29 W/m²Klimit 0.30 |
| EW | Existing Single-Leaf Garage Wall — Insulated Stud Lining | 0.24 W/m²Klimit 0.30 |
| EW | Existing Cavity Wall — Insulated Plasterboard | 0.29 W/m²Klimit 0.30 |
| EW | Existing Garage Wall Shared with the Neighbour | 0.27 W/m²K / REI 60 |
| Internal walls | ||
| IW | New Wall Between Retained Garage and Habitable Room | REI 30 |
| Ground floors | ||
| GF | Existing Garage Slab — Insulation Over, Screed Finish | 0.17 W/m²Klimit 0.25 |
| GF | Existing Garage Slab — Insulation Over, Timber Deck | 0.18 W/m²Klimit 0.25 |
| GF | New Ground-Bearing Slab — Insulation Below Slab | 0.17 W/m²Klimit 0.18 |
| Roofs | ||
| RF | Existing Flat Roof — Warm Deck Overlay | 0.15 W/m²Klimit 0.16 |
| RF | Existing Flat Roof — Insulation Between Joists (Cold Roof, Covering Retained) | 0.15 W/m²Klimit 0.16 |
| RF | Existing Pitched Roof — Insulation at Rafter Level (Room in Roof) | 0.15 W/m²Klimit 0.16 |
| RF | Existing Pitched Roof — Insulation at Ceiling Level | 0.15 W/m²Klimit 0.16 |
| RF | New Flat Roof — Warm Deck | 0.15 W/m²Klimit 0.15 |
20 of 20 carry a calculated U-value. A dash means the element has no thermal target: an internal wall, an intermediate floor, a foundation.
Part B · general notes
36 notes across 18 categories
Arranged in the order a building control officer reads a pack. One subject, one note: nothing repeats and nothing contradicts. Notes about a feature the project may not have — Off-Mains Foul Drainage — Treatment Plants, Septic Tanks and Drainage Fields, Chimneys, Flues, Hearths and Stoves (Part J) — start off, and the review step names every one left off.
- Planning & Permitted Development
- Applicability
- General Notes
- Structure
- Foundations & Infill
- Ground Floors
- External Walls
- Party Wall
- Additional Notes for Walls
- Roofs
- Internal Works
- Means of Escape & Fire Regs
- Windows & Doors
- Ventilation
- Condensation (C2)
- Drainage
- Services
- Conservation of Fuel & Power
Straight answers
Questions about a garage conversion specification
- Does a garage conversion need Building Regulations approval?
- Converting a garage to habitable use is normally a material alteration and needs an application, whether or not planning permission is required. The specification carries an applicability category that states which requirements the conversion brings into force.
- What U-value must the existing garage wall achieve?
- A retained wall being upgraded is a renovated thermal element. Table 4.3 of Approved Document L Volume 1 sets a threshold of 0.70 W/m²K, above which a cavity wall is to be improved to 0.55 and any other wall to 0.30. The new infill wall is a new element and meets 0.18.
- Is the floor a new element?
- A new insulated floor laid over the existing slab is treated as a new thermal element and meets 0.18 W/m²K. The specification carries both the insulation-over-slab and the insulation-below-slab build-ups, with the working for each.
Issue the garage conversion specification under your own name
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