In relation to this popular q, I don't recall seeing riders in EU legislation or the matter even being discussed in relation to it, so I'm guessing some form of single-subject rule is enforced even if it's not explicit. But I cannot find any further discussion on this regarding EU legislation (and I don't mean national legislation of member countries). So, are there such anti-rider provisions in EU legislation/treaties or maybe in EU Parliament procedures?
My own searching leads me to multiple articles (example) pointing to the exact same instance of a legislative rider, and I can't find any others. Basically, in the regulation establishing Member State fishing quotas for 2007, there's a clause allowing limited fishing by electric current, a normally forbidden practice in EU waters (for the legislative source text, see here Annex III section 4, on page 178).
Fishing regulation is nowhere near a subject area I would consider myself knowledgeable, but I'll just note it's debatable whether adding regulation of fishing methods to a law on fishing quotas could be considered a rider.
Other than that, I cannot find a reference to this in the treaties (see TFEU Chapter 2 for the EU legislative process, especially Article 294 for the ordinary legislative procedure used in most cases). So while it seems to be technically allowed, I would argue the decentralized structure of the ordinary legislative process makes riders nearly impossible in practice for three reasons:
- Unusually for Western political set-ups, the legislative process is formally initiated by the executive body of the EU, the European Commission. In theory, the Commission is supposed to "defend the interests of the EU as a whole," arguably riders would normally be counter-productive to smooth running of the EU.
- After the Commission proposes the legislation it must be approved by two further institutions, the Council and the Parliament. Either might object to the rider or potentially just amend it out (which sometimes requires unanimity in the Council though). In theory, this means the legislative process would be much smoother with focussed legislation. Note that even for related subject matter, controversial aspects could risk failing to pass uncontroversial aspects, eg. the controversy surrounding Articles 15 & 17 of the DSM Copyright Directive in an otherwise uncontroversial directive.
- It is possible the rider is in a subject area requiring unanimity in the Council thus making the legislation as a whole more difficult to pass. This is less relevant these days as the Treaty of Lisbon moved most subject areas to qualified majority voting, but it could have played a more prominent role in the past.