do I as a small business also have to complete an ESG questionnaire from my customer

Yes, usually — but not because the law requires it of you

Yes, as a small business you often receive an ESG questionnaire anyway, and usually it makes sense to complete it. Not because the CSRD (Directive (EU) 2022/2464) obliges you as a small undertaking to report — it does not — but because your customer is subject to the CSRD or will be subject to it, and is required to map his supply chain. Your answers are input for him for scope 3: the emissions and risks that do not come from his own business operations, but from those of his suppliers. If you refuse, that question does not disappear — your customer will then fill in something himself, with an estimate instead of your figures.

The proof document that is usually requested is not a certificate but a dataset: number of employees, energy consumption, possibly CO2 emissions per scope, and sometimes a reference to your annual turnover as context for those figures. If you don't have that data readily available, then 'not yet available' is a more valid answer than an estimate you cannot substantiate — especially because the same list comes back next year and a substantiated 'no' holds up better than a poorly founded 'yes'. Increasingly, reference is made to the EFRAG VSME: the voluntary basic module specifically designed for non-listed SMEs, with a limited, manageable set of indicators instead of the full CSRD reporting.

Where this is based on: CSRD supply chain obligation and VSME basic module

The obligation to report on the supply chain lies with the large undertaking, laid down in Directive (EU) 2022/2464; that obligation flows through to suppliers without the directive itself prescribing anything to them. The EFRAG VSME basic module is the voluntary, simplified standard specifically created to give SMEs a manageable answer to that kind of questions, without the full CSRD burden. Incidentally, what you include in such a list about where your business operates can also be relevant to the question in which countries the enterprise has establishments, because some customers treat that as a separate part of the ESG list.