Marking a product is not a test you pass at the end. It is a file you build as you go, and a declaration somebody in your company signs and is answerable for. Treating it as a final hurdle is the single most expensive habit I see in SME hardware projects.
Work out which rules apply, on day one
A connected product is rarely covered by one directive. A mains-powered industrial sensor with Bluetooth and a display typically sits under the Radio Equipment Directive, the EMC Directive where the RED does not already absorb it, the Low Voltage Directive, RoHS, and — increasingly — the Cyber Resilience Act. Battery-powered, radio-only devices fall largely under RED, which pulls in EMC and safety through its own essential requirements.
For the UK you need the same evidence again under UKCA, referencing the equivalent UK statutory instruments. In practice the technical work is shared; the paperwork is duplicated. Plan for both from the start rather than rebuilding the file later.
The decision that matters most is whether you can self-declare against harmonised standards or need a notified body. Use the harmonised standards where you can — a device built to EN standards listed in the Official Journal gets presumption of conformity, and that is the cheap path. Deviate from them, and you are into a notified body assessment and a schedule measured in months.
What actually goes in the technical file
A technical file is not a folder of certificates. For a connected product it should hold: the product description and intended use; schematics, layout and a bill of materials; the risk assessment; the list of standards applied and how each requirement is met; test reports, including any pre-compliance work; user documentation and the labelling artwork; the radio module's own approvals and the conditions attached to them; and the signed declaration of conformity.
The part most often missing is the reasoning: which standard, which clause, which piece of evidence. An assessor asked to reconstruct your logic from a pile of PDFs will take longer and charge more. A one-page mapping table makes the difference.
Three mistakes that cost months
Assuming a pre-approved radio module carries you. It does not. A certified module reduces the testing burden, but only if you follow its integration conditions exactly — antenna type and gain, ground plane, keep-outs, trace geometry, firmware power settings. Change the antenna and you may have inherited a full radio test campaign. I have seen a board layout invalidate a module approval three weeks before launch.
Leaving the enclosure and the label to the end. Markings, ratings, warnings and the mark itself have placement and legibility requirements. Tooling an enclosure before that is settled means either a compromise sticker or a tool modification.
Not naming the person who signs. The declaration of conformity is signed by someone at the manufacturer, on behalf of the company. It carries legal weight. If nobody has agreed to be that person, the file drifts, because no one owns it.
Where an engineer earns their fee
The useful work happens early: deciding the compliance route before the schematic is frozen, choosing a module whose conditions your layout can actually meet, and booking pre-compliance at the point where a failure is still cheap to fix. The paperwork is tedious but straightforward once the technical decisions are right. It is the technical decisions made blind that turn into six-figure surprises.
If you are staring at a product that needs marking and you are not sure which route applies, that is a half-day conversation, not a project. It is usually worth having before the layout is finished.