Quick answer: new walls 0.28, floors 0.22, roofs 0.16 insulated at ceiling level or 0.18 at rafter level or flat (Table 3.2); windows, roof windows and rooflights 1.6 and doors 1.8 (Table 3.1). Openings are limited to 25% of the floor area plus openings no longer exposed (para 3.11). Over that, or with an element above its value, the extension complies if its area-weighted U-value is no worse than a notional extension's (para 3.14); failing that, a SAP calculation for the whole dwelling (para 3.15).
Free to check. Building Control-ready PDF £35 +VAT (£42 inc VAT).
Runs in your browser. Pick the roof type under the roof U-value — it sets the Table 3.2 value.
The U-values: Tables 3.1 and 3.2
| Element | Northern Ireland, TB F1 (W/m²K) | England, ADL1 Table 4.2 |
|---|---|---|
| External walls | 0.28 | 0.18 |
| Pitched roof, insulation at ceiling level | 0.16 | 0.15 |
| Pitched roof at rafter level, flat roof, or integral insulation | 0.18 | 0.15 |
| Floors | 0.22 | 0.18 |
| Windows, roof windows and rooflights | 1.6 (or WER band C) | 1.4 (rooflights 2.2) |
| Doors | 1.8 | 1.4 |
Roof windows and rooflights are assessed in the vertical position. Use the manufacturer's whole-unit U-value, not the centre-pane figure.
Three ways to comply (para 3.8)
- Standards based (paras 3.10–3.13): every element to Table 3.1 or 3.2, and windows, roof windows, rooflights and doors no more than 25% of the floor area plus the area of openings that no longer exist or are no longer exposed because of the extension (para 3.11).
- Calculated trade-off (para 3.14): the area-weighted U-value of all the elements of the extension — Uav = Σ(U×A) ÷ ΣA — no greater than that of an extension of the same shape and size that meets paras 3.10 and 3.11. Elements may be traded against each other, and so may glazing over the 25%.
- Equivalent carbon target (paras 3.15–3.16): SAP shows the house with its extension emits no more CO₂ than the house with a notional extension — its door area equal to yours, the rest of the 25% as windows. Upgrades to the existing house can count; the upgraded parts are then treated as retained elements.
How the notional extension is built
Para 3.14 describes the notional extension as the same shape and size, meeting the para 3.10 U-values and the para 3.11 glazed area. GlazeMonkey builds it the standard way: your openings kept in proportion and scaled down to the allowance, the excess returned to wall (vertical openings) or roof (rooflights) at their Table 3.2 values, and every element at its table value — windows, roof windows and rooflights 1.6, doors 1.8.
Worked example
A 20 m² rear extension with a flat roof: bifold doors (7.56 m²), a window (1.44 m²) and a rooflight (1 m²), all at 1.4, so 10 m² of openings against an allowance of 25% × 20 + 1.8 = 6.8 m² (a 1.8 m² back door is no longer exposed). Walls 30 m² gross at 0.22, roof 20 m² at 0.13, floor 20 m² at 0.18.
| Element (table value) | Proposed A | U | U×A | Notional A | Notional U×A |
|---|---|---|---|---|---|
| External walls (0.28) | 21.00 | 0.22 | 4.62 | 23.88 | 6.69 |
| Flat roof (0.18) | 19.00 | 0.13 | 2.47 | 19.32 | 3.48 |
| Floor (0.22) | 20.00 | 0.18 | 3.60 | 20.00 | 4.40 |
| Bifold doors (1.8) | 7.56 | 1.40 | 10.58 | 5.14 | 9.25 |
| Window (1.6) | 1.44 | 1.40 | 2.02 | 0.98 | 1.57 |
| Rooflight (1.6) | 1.00 | 1.40 | 1.40 | 0.68 | 1.09 |
| Total / area-weighted U | 70.00 | 0.353 | 24.69 | 70.00 | 26.47 → 0.378 |
The notional extension keeps 68% of each opening type (6.8 ÷ 10), so 2.88 m² of the vertical glazing goes back to wall at 0.28 and 0.32 m² of rooflight back to roof at 0.18. The proposed extension's area-weighted U-value is 0.353 against the notional 0.378 — 6.7% better, so it complies by para 3.14 despite being 50% glazed. Put 1.6 units in instead of 1.4 and it is 0.381 against 0.378: a fail, by 0.8%.
Conservatories and porches
- Exempt (Schedule 2, Class 8; para 3.17). At ground level and no more than 30 m² — a porch no more than 5 m² — while the thermal performance of the house is retained, its heating or cooling system is not extended into it, no fixed combustion or cooling appliance is installed, and the glazing satisfies Part V.
- Not exempt, kept thermally separated (para 3.19). Windows and doors to Table 3.1, new walls, roof and floor to Table 3.2, no limit on the glazed area, and independent temperature and on/off controls for any heating or cooling. Para 3.20 also allows it to be assessed as an extension instead.
- Open to the house: it is an extension, with the 25% limit.
A conservatory has at least three-quarters of its roof and half of its external walls translucent. If an exempt one later loses its separation, or has the heating extended into it, it becomes a change of energy status and has to comply.
How GlazeMonkey does it
- Choose Northern Ireland under "Where is the dwelling?" in step 1 (the button above does it for you).
- Enter the floor area, the openings, the openings no longer exposed, and the gross wall, roof and floor areas with their U-values — and pick the roof type, which sets the roof value.
- The result shows the table checks, the allowance and the para 3.14 comparison, live. If it fails, "How to make it comply" lists the changes that pass, each re-checked.
- The PDF — a Part F compliance report — shows every figure, the notional extension line by line, and cites the paragraphs it used.
Frequently asked questions
What U-values does an extension need in Northern Ireland?
Technical Booklet F1 (June 2022) asks for new walls at 0.28, floors at 0.22, and roofs at 0.16 where the insulation is at ceiling level or 0.18 at rafter level, on a flat roof or with integral insulation (Table 3.2). Windows, roof windows and rooflights should be 1.6 W/m²K or WER band C, and doors 1.8 (Table 3.1).
Does the 25% glazing rule apply in Northern Ireland?
Yes. Para 3.11 limits the windows, roof windows, rooflights and doors of an extension to 25% of its floor area, plus the area of any windows, roof windows, rooflights or doors that no longer exist or are no longer exposed because of the extension.
What if my extension has more glazing than that?
Para 3.14 allows it if the area-weighted U-value of all the elements of the extension is no greater than that of an extension of the same shape and size that meets the U-values in para 3.10 and the glazed areas in para 3.11. Failing that, para 3.15 uses SAP to show the house with its extension emits no more CO₂ than the house with a notional extension.
Is a conservatory exempt in Northern Ireland?
Yes, under Schedule 2, Class 8, while it is at ground level and no more than 30 m² (a porch, 5 m²), the thermal performance of the house is retained, the heating or cooling system is not extended into it, no fixed combustion or cooling appliance is installed, and the glazing satisfies Part V. A non-exempt one kept thermally separated is assessed under para 3.19, with no limit on its glazed area.
Why are the Northern Ireland values higher than England's?
Technical Booklet F1 was last revised in June 2022, and its values for work to existing dwellings — walls 0.28, windows 1.6 — are older and less demanding than England's 2021 Approved Document L (walls 0.18, windows 1.4). Which applies depends only on where the house is.
Sources
- Department of Finance, Technical Booklet F1: Conservation of fuel and power in dwellings (June 2022) — Section 3, paras 3.8–3.20 and 3.29, Tables 3.1 and 3.2.
- The Building Regulations (Northern Ireland) 2012, Schedule 2 — Class 8, conservatories and porches.
Also: the rules in Scotland · England's area-weighted method · openings the extension covers over.