Publish the registration
The word itself proves nothing.
"Engineer" is unprotected in the UK. CEng and IEng are protected, and verifiable.
In most of Europe the title engineer is protected and its use restricted to those who have qualified. In the United Kingdom it is not. Anyone may use the word, in any field, with any level of training or none at all, and no offence is committed. What is protected is registration with the Engineering Council — Chartered Engineer, Incorporated Engineer, Engineering Technician — awarded through licensed institutions after assessment, and listed on a register anyone can check.
The practical consequence is that the single most reassuring word in the field conveys no information. A client comparing two consultants on the basis that both describe themselves as engineers is comparing nothing. So the useful question is narrower and entirely answerable: what registration is held, with which institution, and can it be verified. We publish ours and we would encourage the same question of anyone else, because a claimed designation that does not appear on the register is a serious matter rather than a formality.
Competence has also stopped being purely a professional norm. The regime now applying to building work subject to Building Regulations control places duties of competence on clients, designers and contractors, and requires a designer not to accept an appointment for work they are not competent to carry out, with heightened requirements for higher-risk buildings. Declining a commission is therefore part of doing the job properly. The related boundary matters just as much: we are not a building control body, we do not certify compliance or statutory approval, and an opinion from us says so on its face.
The word itself proves nothing.
A duty, not a preference.
An assessment is valid only on its basis.
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