What design temperature should I use for an office?

Typical winter heating setpoint is 21°C for open-plan offices. Summer cooling where provided is often 24–26°C — confirm against the project brief and CIBSE Guide A.

How do I calculate room heat loss?

Sum fabric losses (U × A × ΔT) plus ventilation loss from air change rate and room volume. Use design external temperature for your UK location.

What U-values should I use for a heat loss check?

Use the architect's issued U-values from the specification. Part L notional values are for screening only — not for final plant sizing.

BuildFlo Design Guide · Thermal · CIBSE Guide A / Part L

Temperature design for UK building services

Design temperatures for UK offices, classrooms and plant rooms. Built-in room heat loss calculator with Part L fabric U-values.

Choosing the right design temperature is one of the first decisions on any heating or cooling job. Get it wrong and you oversize plant, miss comfort targets, or fight the client over setpoints later.

This guide lists typical internal design temperatures by room type, then walks through a single-room heat loss using Part L notional fabric U-values - the sort of first-pass check you do before opening a full SAP or dynamic model.

Design temperatures by room type

Pick a room in the calculator below to apply its design temperature automatically. Values are indicative - confirm against the project brief, UK design guidance, BB101 (schools), or HTM (healthcare).

Room type	Category	Winter heating	Summer ( cooled )	Typical ACH
Living room ( dwelling )	Domestic	21°C	-	0.5
Bedroom ( dwelling )	Domestic	18°C	-	0.5
Kitchen ( dwelling )	Domestic	21°C	-	1.0
Bathroom ( dwelling )	Domestic	22°C	-	1.5
Open-plan office	Commercial	21°C	25°C	1.0
Meeting room	Commercial	21°C	24°C	1.0
Reception / foyer	Commercial	19°C	26°C	0.8
Corridor / circulation	Commercial	17°C	-	0.5
WC / changing	Commercial	19°C	-	1.5
Classroom	Education	20°C	26°C	1.0
Retail sales	Retail	20°C	25°C	1.0
Sports hall	Leisure	17°C	-	1.5
Plant room	Plant	12°C	-	0.5
Server / comms room	Plant	20°C	22°C	2.0
Store / cupboard	Commercial	16°C	-	0.3

External winter design temperature for this demo: −4°C ( UK screening value - use UK design guidance for your site ). ΔT = internal design temp − external temp.

Room heat loss demo

Enter room dimensions and glazing area. U-values are filled from Part L notional dwelling fabric - adjust on real jobs from the architect's spec.

Room type (sets design temperature)

Design temperature21°C

ΔT ( vs -4°C external )25 K

ACH ( infiltration / ventilation )0.5

Part L / SAP living area

Length (m)Width (m)Height (m)Glazing / window area (m²)

Floor area: 21.3 m²

Auto-filled Part L fabric U-values

Wall0.18 W/m²KRoof / ceiling0.13 W/m²KFloor0.15 W/m²KWindows1.4 W/m²K

Wall 0.18 · Roof 0.13 · Floor 0.15 · Window 1.4 W/m²K (notional dwelling). Limits: wall 0.26, roof 0.16, floor 0.18, window 1.6.

Full heat load calculator →

How the heat loss calculation works

1. Fabric loss on each surface: Q = U × A × ΔT ( W )

Walls = gross wall area minus window area
Roof and floor = plan area ( length × width )
Glazing = window area you enter

2. Ventilation loss: Q = ρ × cp × V̇ × ΔT where V̇ = ACH × room volume / 3600

3. U-values in the demo are Part L notional dwelling fabric ( indicative for new-build screening ):

Wall 0.18 · Roof 0.13 · Floor 0.15 · Window 1.4 W/m²K
Part L limiting values ( England 2021 ): wall 0.26 · roof 0.16 · floor 0.18 · window 1.6 W/m²K

On a real project, replace these with the architect's issued U-values from the specification - never size final plant on notional defaults alone.

Extended reference - non-domestic setpoints

Space type	Winter heating	Summer cooling ( where provided )
Open-plan office	21-22°C	24-26°C
Meeting room	21°C	24°C
Reception / foyer	18-20°C	26°C
Corridor / circulation	16-18°C	-
WC / changing	18-21°C	-
Plant room	10-16°C ( frost protection )	Ventilation only
Server / comms room	18-20°C	18-24°C ( equipment-led )
Classroom	20°C ( BB101 )	Passive / limited cooling
Retail	19-21°C	24-26°C
Sports hall	16-18°C	Natural ventilation often sufficient

Domestic dwellings - Part L and SAP assume around 21°C in living areas and 18°C in bedrooms for annual energy modelling.

How to use these on a job

1. Confirm the brief - Cat A vs Cat B, client ESG targets, and BREEAM/WELL may tighten bands. 2. Check the standard - Schools: BB101. Healthcare: HTM/HBN. Offices: UK design guidance Table 1.5. 3. Size on design external conditions - UK design guidance external winter/summer design for your location. 4. Allow for diversity - Not every zone calls simultaneously; apply load diversity sensibly. 5. Document your assumption - Note the design temp on calculations and drawings.

Common mistakes

Using 21°C everywhere including plant rooms, storerooms, and corridors - oversizes the system.
Ignoring metabolic rate in sports halls, gyms, and kitchens - higher internal gains.
Mixing up operative vs air temperature when the client specifies one or the other.
Sizing on Part L notional U-values when the project spec has better ( or worse ) actual build-ups.
Forgetting setback - night/weekend temperatures affect annual energy, not peak plant size.

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*Need the full standard citation? Ask BuildFlo in the app - it indexes UK design guidance and Part L.*

Frequently asked questions

What design temperature should I use for an office?: Typical winter heating setpoint is 21°C for open-plan offices. Summer cooling where provided is often 24–26°C — confirm against the project brief and CIBSE Guide A.
How do I calculate room heat loss?: Sum fabric losses (U × A × ΔT) plus ventilation loss from air change rate and room volume. Use design external temperature for your UK location.
What U-values should I use for a heat loss check?: Use the architect's issued U-values from the specification. Part L notional values are for screening only — not for final plant sizing.