Assuming that the fundamental principles of the Community and the Union need to be preserved, it is necessary to explain in the last step which principles are relevant for the assessment of the admissibility of exceptions and their consequences. One can already state here that the legally relevant principles, namely those of unity of law and non-discrimination, can be themselves subject to exceptions. However, limits that flow from the necessary balancing between the principles in question have to be respected.54
a) First a word about the meaning of unity of law. It is ambiguous. On one hand, it does not, as a formal concept, provide protection against a reduction of generally applicable Community law and, therefore, also not against taking that step upon accession of new Member States. On the other hand, it reduces the possibility of differentiation to the benefit or detriment of individual Member States, provided that the law remains valid for the other Member States and also provided that the admissibility of a differentiation does not follow from the Treaties.55 This second aspect results in exceptions necessitating a justification to be found in another legal principle of the Community, including the fact that the exceptions should, in the end serve, by momentary renunciation, the purpose of creating the unity of law which cannot be achieved immediately.
b) In a certain way, the unity of law just explained is also an expression of a general rule of non-discrimination. This is - especially in relation to the duties and the rights of the Member States - a part of Community law. Equal participation in the integration process and uniform applicability of the Treaty provisions aimed at it do not follow from the prohibition of discrimination in Art. 12 E.C. Treaty56 but from a general rule of law.57 This general principle plays an important role in the enlargement of the Union, because one can deduce that new Member States have to have the same rights and duties as the old Member States.58 Still, as a principle of general character, the principle of non-discrimination does not exclude any differentiation but restricts exceptions from the application of Community law by imposing the requirement that there has to be a reasonable justification for it.59
c) For exceptions from the uniformly applicable Community law, especially for the agreement of transitional measures, justification in the form of an objective reason is needed. This could be based on the endangerment of other legal values protected by Community law. According to the general aims of European integration, a geographical extension of the European Union and the European Communities, and with that an extension of Community law, is not only politically desirable but also a legally protected value. In connection with an accession, there are special circumstances to be taken into consideration which favor a relatively wide discretion marked by the political character of such a fundamental decisions. A decision to accept new Member States is characterized by its complexity. Besides economic advantages and disadvantages, there are also strategic and geopolitical aspects to be taken into account leading to the constellation that, although conditions of an accession can be controlled, the evaluation of the enlargement itself is only subject to a limited control.60 In sum, deviations from the unity of law and the rule of non-discrimination are to be tolerated if they serve, in the end, the purpose of a per se admissible accession and aim at simplifying the mutual adaptation and securing unity and equality in the whole Community area. These functions of adaptation and security explain why transitional measures could be agreed upon at earlier accessions and why such measures are also uniformly held admissible in the literature.
From the outlined general foundation one can deduce certain concrete principles which form, with the necessity of judging every particular case on its own merit, a certain framework in which certain possibilities of justified exceptions exist. In practice, this means weighing the legal values involved and putting them into relation.
a) If the fundamental values of the Community itself are affected,61 discrimination of a permanent character between the Member States is, in principle, not possible.62 Amongst these fundamental values are regulations concerning the internal market and also rules concerning the four basic freedoms, such as the free movement of workers and of services.
b) For a limited period of time, exceptions are admissible to all rules of Community and Union law. However, transitional measures may not result in a permanent non-application of law in areas which define the Community's identity. Therefore, transition periods have to be chosen in a way so that unity of law can be expected in the not too distant future. Their duration can depend on the measure in which Community law is affected at its core, and also on the extent of the expected adaptation problems. It also has to be taken into account how costly the application of transition measures will be.63 As a guideline one can assume, looking at the practice so far, that the maximum duration for a transition in areas which affect fundamental rights lies at approximately ten to twelve years.64
c) Apart from measures limited only by time, in exceptional situations measures also limited in substance can be admissible. This means regulations which only give rights if certain conditions are fulfilled - such as the introduction of free movement of workers only after achieving a certain degree of economic adaptation. However, limited transition measures need to be justified by exceptional measures, and an assurance that the creation of a permanent exception is prevented. Therefore, substantial limitations may only be agreed upon if, in addition, a certain time limit is also affixed.
Do circumstances at accession lead to the conclusion that (obviously) the candidates for accession will not succeed, within the admissible time limit, to adapt sufficiently to the economic level of the EU? - Are dangers, after an expired time limit, for the labor market of the old Member States or other serious collisions of legal values to be expected65 if free movement of workers and services are granted to the full extent if the enlargement of the EU is to be regarded as against Community law? When it becomes clear that fundamental principles of the Community are to be destroyed by permanent differentiation or by endangering the achieved level of integration, discretion as to the conditions for an accession is limited.
54 Also insofar see more detailled Becker, EU-Erweiterung und differenzierte Integration (footnote 19), p. 73.
55 Compare above III.1.
56 So Langeheine, Abgestufte Integration, EuR 1983, pp. 227, 239; also see Kapteyn/VerLoren van Themaat, Introduction to the Law of the European Communities, 3rd ed. (1998), p. 168 et seq.; for art. 12 E.C. Treaty as basis Coen, Abgestufte soziale Integration nach Maastricht, EuZW 1995, p. 52.
57 Footing in general arguments Ipsen, Europäisches Gemeinschaftsrecht (1972), 9/34; for a connection between modified integration and loyalty to the Community see Bleckmann, in: Bleckmann, Europarecht, 6th ed. (1997), no. 706.
58 At least in regard to area-related differentiations it should be sufficient for a discrimination that some Member States are willing to pursue integrations, others however not, see Bleckmann, in: Bleckmann, Europarecht, 6th ed. (1997), no. 706. This thought however is not of relevance when the application of already existing, not the creation of new Community law is at stake.
59 See Ehlermann, How flexible is Community law? An unusual approach to the concept of "two speeds",  MichLRev., p. 1274, 1288 et seq.
60 Also see cases 31 and 35/86, LAISA  ECR 2285/15, and infra, VI.
61 See above, 3.b)
62 Skeptical, but without differentiation as to legal areas Bieber, in: Beutler/Bieber/Pipkorn/Streil, Die Europäische Union, 4th ed. (1993), p. 586; opposing are also those opinions in the literature that see, at an accession, transition regulations as admissible and underline the temporary character of those regulations; also see Klein, in: HandKom. EUV/EGV, Art. O EUV, no. 20. Most authors do not give further reasons.
63 As already mentioned, e.g. the re-introduction of border controls would be made impossible by the necessary expenditure, because at the same time, all other basic freedoms in the internal market would be affected; regularly, temporary exceptions can be realized by national measures and their control by general supervision, so controls of the internal economic activities.
64 Preston, Enlargement and Integration in the European Union (1997), p. 233, thinks a longer possibility (up to 20 years) theoretically possible, but practically impossible.
65 This means not simply difficulties at the economic integration: those are supposed to be helped by the legal order of the Community.