Sector
Commercial offices
MEP design for new build and refurbished office buildings, from shell and core through to Cat A and Cat B fit-out.
OVERVIEW
Office MEP design divides into shell and core, Cat A and Cat B, and the division matters because it determines who carries the risk of the assumptions.
Shell and core sets incoming capacity, plant space and riser provision against an assumed occupancy density. If the eventual tenant density exceeds that assumption, the cost of the shortfall lands on someone — and it is worth being explicit at design stage about what density the building has been designed to.
DESIGN
What drives the design
Occupancy density is the driving assumption
Everything downstream — fresh air rate, cooling load, small power capacity, sanitary provision — derives from occupancy density. The British Council for Offices guidance gives densities for design purposes, and the figure used should be stated on the drawings rather than buried in a calculation.
Small power has grown and then shrunk
Small power loads per person have fallen substantially as desktop machines gave way to laptops, but the diversity has changed shape rather than simply reduced. Designing to a legacy W/m² figure oversizes the distribution and the cooling that removes the heat from it.
Flexibility has a cost that should be priced
Speculative offices are frequently asked to accommodate any future layout. That is achievable, but it is a decision about capital cost and plant space, not a free property of a good design.
REGULATION
Statutory and guidance documents
Scotland and England take different statutory routes. Where a document applies to one and not the other, it says so.
- Approved Document LConservation of fuel and powerEngland and Wales
- Technical Handbook Section 6EnergyScotland
- Approved Document FVentilationEngland and Wales
- BS EN 12464-1:2021Lighting of indoor work placesUK-wide
- BCO Guide to SpecificationBritish Council for OfficesUK-wide
SCOPE
Typical scope in this sector
- Shell and core MEP design
- Cat A fit-out design
- Cat B tenant fit-out design
- Incoming supply capacity assessment
- Landlord and tenant metering strategy
- EPC and Part L or Section 6 compliance
QUESTIONS
Common questions
What is the difference between Cat A and Cat B office fit-out?
Cat A is the landlord’s fit-out — raised floors, suspended ceilings, basic lighting, and mechanical and electrical distribution to a lettable standard. Cat B is the tenant’s fit-out, adapting that base to their layout, with meeting rooms, tea points, IT and any additional cooling or power their occupancy needs.
What occupancy density should an office be designed to?
The British Council for Offices guidance is the usual reference, and design densities in the range of one person per 8 to 12 square metres of net internal area are typical for open plan space. The density assumed should be stated explicitly, because fresh air rate, cooling load and sanitary provision all derive from it.
SERVICES