CDM Regulations Construction Guide
"CDM only applies to big commercial sites" is one of the more expensive myths in UK construction. The Construction (Design and Management) Regulations 2015 apply to every construction project in England, Scotland and Wales, domestic loft conversion included. What changes on a domestic job isn't whether CDM applies, it's who ends up holding the client duties. Get that wrong and the costs it should have covered show up as a surprise mid-build instead of a line in the original estimate.
Key Takeaways
- CDM 2015 assigns duties to five roles: client, principal designer, principal contractor, designers and contractors, and applies to every UK construction project, not just commercial sites.
- A project is notifiable to the HSE if it runs longer than 30 working days with more than 20 workers on site at once, or exceeds 500 person-days total.
- On domestic projects, the client duties transfer to the contractor (or principal contractor on a multi-contractor job) by default.
- The Construction Phase Plan, Health and Safety File and trade-specific RAMS are the three documents CDM requires, and each carries a cost most estimates handle badly.
- CDM-driven prelims, welfare, site management, principal contractor overhead, deserve their own bill lines, not a vague percentage buried in the total.
The five duty holders
CDM 2015 splits responsibility across five roles, and every UK construction project has all five filled by someone, even when nobody's formally named the role.
| Duty holder | Core duty |
|---|---|
| Client | Sets project parameters, appoints competent designers and contractors, notifies HSE where required |
| Principal Designer | Plans pre-construction safety, identifies design-stage risk, compiles the Health and Safety File |
| Principal Contractor | Manages construction-phase safety, writes the Construction Phase Plan, coordinates contractors and welfare |
| Designers | Eliminate or reduce foreseeable risk through design, across build, maintenance and demolition |
| Contractors | Plan and manage their own work safely, produce trade-specific RAMS, ensure worker competence |
Principal Designer and Principal Contractor only get formally appointed on projects with more than one contractor. On a single-contractor job, that one contractor absorbs both the contractor and principal contractor duties automatically.
When a project becomes notifiable
A project needs HSE notification, using Form F10, once it crosses either of two thresholds: more than 30 working days with more than 20 workers on site at the same point, or more than 500 person-days of construction work total. Notification happens before work starts on site, not after.
Neither threshold is huge. A mid-sized extension with several trades overlapping can brush against the 500 person-day figure faster than people expect, groundworkers, then the frame, then first fix, then finishes, all adding days. Estimate a project without checking this and you can miss the F10 requirement entirely, which is a compliance gap, not just a paperwork one.
Domestic clients and CDM
Homeowners are still clients under CDM, full stop, but the regulations recognise they rarely have construction expertise. So the client duties transfer automatically: to the contractor on a single-contractor job, to the principal contractor where there's more than one. The homeowner doesn't need to run risk assessments themselves. Somebody still has to, and that somebody's cost belongs in the tender.
This is exactly where domestic estimates go wrong most often. Everyone assumes CDM is a commercial-site problem, prices the job as if it isn't there, and then the contractor discovers mid-project that welfare facilities, a written Construction Phase Plan, and proper site management weren't actually optional. For the wider regulatory picture beyond CDM specifically, our UK construction standards and regulations guide covers how it sits alongside Building Regulations and NRM2.
CDM applies to a loft conversion the same way it applies to a tower block. What changes is who carries the client duties, not whether they exist.
The three documents CDM requires
The Construction Phase Plan sets out how the site's managed safely, welfare arrangements, site rules, emergency procedures, before work starts. It's a live document, updated as the project develops, not a one-off form filed and forgotten.
The Health and Safety File is the record that outlives the project, what a future contractor, maintenance team or demolition crew needs to know about the building's construction, materials and hazards. Skip it and every future piece of work on that building starts from zero information.
RAMS, risk assessments and method statements, sit at the trade level. Every contractor produces their own, specific to their actual work on that specific site, not a generic template swapped in from the last job.
None of these three documents write themselves for free. Producing and maintaining them takes hours, and those hours belong somewhere specific in a cost plan, not folded invisibly into a contractor's overhead.
Where CDM costs actually belong in an estimate
This is the part most guides to CDM skip entirely, they explain the regulations and stop before the money. Four cost categories trace directly back to CDM duties, and each one deserves its own bill line rather than a vague prelims percentage.
Welfare facilities. Toilets, washing, rest areas, drinking water, from day one of site occupation, not once someone gets around to it. Site size and duration set the cost.
Site management time. Whoever holds principal contractor duties spends real hours coordinating trades, running inductions, and keeping the Construction Phase Plan current. That's a genuine labour cost, not free overhead.
Principal Designer input. On multi-contractor jobs, someone gets paid to hold this role properly, identifying design risk and compiling the Health and Safety File. Absorbing it silently into a design fee without pricing it separately is how it gets skipped under time pressure.
Temporary works coordination. Scaffolding, propping, excavation support, all need CDM-compliant planning and sign-off, and that coordination has its own cost separate from the physical kit.
We break these out individually in every estimate rather than burying them, so a contractor tendering on our numbers knows exactly what they're pricing and why. It's the difference between a competitive number and one that quietly falls apart once the real prelims cost shows up mid-project.
Frequently asked questions
Don't let CDM prelims surface as a surprise mid-project.
Send us your drawings. We'll price welfare, site management and documentation as clear, separate lines, in 10-48 hours.
Looking for support across the full project, not just CDM? Our construction estimating services cover residential, commercial and trade projects UK-wide.