Energy supply and infrastructure

multidisciplinary consulting.

The multidisciplinary energy and infrastructure team at SONNTAG has been supporting private and public clients in the successful implementation of energy and infrastructure projects for twenty years. Renewable energies, district heating supply and water management are our focal points.

According to the motto "law and calculation", we support all project phases in a business management and legal context. From the initial idea, through preparation and implementation, right up to ongoing operation.

Together with you, we discuss the conception of the projects from an economic and legal point of view. We accompany you through subsidy procedures and project financing, develop and negotiate the necessary contracts and provide support in approval procedures. Many years of experience in bidding and EU-wide award procedures round off our profile.

To ensure the success of the project, we act in an economically and legally holistic manner. We combine in-depth knowledge of the industry with sustainable interfaces with engineers, technicians and geologists resulting from many years of cooperation. Whenever it becomes necessary in the course of a project, we also conduct court proceedings, such as antitrust disputes over supply prices.

We promote climate protection out of conviction and actively shape the energy and heat turnaround for Germany together with our clients. Together we identify potential for the use and expansion of renewable energies. We are leaders in advising on the use of deep geothermal energy for heat supply and power generation. The geothermal expertise of the SONNTAG team stems from a large number of successfully implemented and advised projects in this specialised sector.

Since 2004, the team has implemented projects with a thermal capacity of around 200 MW and an investment volume of more than 500 million Euros in its central function as business and legal advisor. SONNTAG's energy and infrastructure team therefore has a broad advisory spectrum.

employment contract | termination.

The employment relationship is a defining legal relationship for every company. Errors and disruptions in this legal relationship can give rise to significant negative consequences. As a result, employment contracts, but also their termination, are of great importance for every company.

subject areas.

  • Employment contracts
  • Termination and settlement agreements
  • Terminations and termination agreements
  • Defending unfair dismissal suits


  • Preparing and reviewing/revising employment contracts
  • Preparing supplemental agreements for employment contracts (company car regulations, bonus and target agreements, commission agreements, agreements for the use of communication devices, or BYOD)
  • Strategic consulting for the purpose of terminating employment relationships
  • Creating termination notices and counsel on the appropriate participation of the works council, as well as within the framework of agency assent procedures
Prof. Dr. Andreas Katzer
  • Partner | Lawyer, M.I.L (Lund)

works constitution and collective bargaining law.

With multidisciplinary engagement and closely involved IT expertise, we offer our clients full support for all collective bargaining law questions and structures.

subject areas.

  • Co-determination rights of the works council
  • Works council elections
  • (In-house) collective bargaining agreements and operational regulations
  • Work environment 4.0
  • Data protection and employee rights (monitoring, utilizing evidence at the time of termination)


  • Consulting on all works constitution legal questions
  • Creating works agreements and (in-house) collective bargaining agreements
  • Negotiations with works councils and unions
  • Representation in disputed cases before the labor court and the conciliation board

employing outside personnel | hiring out employees.

Misjudgments in employing outside personnel can have particularly negative consequences for companies. We ensure expert development of all legal relationships with subcontractors, freelance employees, and with regard to the subject of hiring out employees.

subject areas.

  • Freelance work and fictitious self-employment
  • (fictitious) contracts for work (fictitious) service contracts and impermissible hiring out of employees


  • Creating contracts and approaches
  • Review of social insurance law status
  • Conducting status determination proceedings, including subsequent appeal and court proceedings
  • Applying for permits according to the German Law on Temporary Employment (AÜG)
  • Counsel and implementation in cases of permissible hiring out of employees
  • Delimitation of the various forms of employing outside personnel in national and international contexts
Prof. Dr. Andreas Katzer
  • Partner | Lawyer, M.I.L (Lund)

social insurance law | status determination.

Sonntag & Partner has remarkable expertise in all social insurance law questions within established multidisciplinary teams. We also advise and represent our clients in particularly complex factual circumstances.

subject areas.

  • Status assessment
  • Company audits
  • Social insurance contributions for artists
  • Pensions
  • Social insurance exemptions for managers


  • Counsel and external representation
  • Representation towards social insurance carriers and other regulatory authorities
  • Conducting application and appeal proceedings
  • Reviews of social insurance status under the law
  • Evaluation of contribution obligations within the scope of social contributions for artists
  • Providing support in the course of company audits by pension insurance carriers, including a subsequent appeal and proceedings before the social courts
Prof. Dr. Andreas Katzer
  • Partner | Lawyer, M.I.L (Lund)

representation before the labor courts.

We comprehensively represent our clients in all labor court proceedings in all instances.

subject areas.

  • Disputed terminations of employment relationships
  • Disputed (payment) claims arising from employment relationships
  • Contested resolution of disputes under works constitution law
  • Proceedings for/against boards of directors and managers


  • Attending scheduled court dates in all instances throughout Germany
  • Evaluation of the prospects for success for lawsuits and appeals
  • Judgment proceedings
  • Labor court proceedings
  • Interim legal protection
  • Appellate proceedings (complaint, appeal, points of law)
Prof. Dr. Andreas Katzer
  • Partner | Lawyer, M.I.L (Lund)

Wages | Salary

  • Running payroll and travel expense reports
  • Administration of vacation entitlements
  • Certification system
  • Payroll tax and social security (clarification of payroll tax and social security issues, assistance with payroll tax and social security audits)
  • Management of a digital personnel file
Daniel Schild
  • Team leader | wages and salaries

criminal law.

Criminal law consulting and representation at all stages prior to, or during a payroll tax investigative and criminal proceeding that involves employment relationships under social insurance law.

subject areas.

  • Withholding and misappropriating compensation according to § 266(a) Criminal Code (StGB) (illegal employment/fictitious self-employment)
  • Fraud in making contributions according to § 263 StGB
  • Illegal hiring out of employees


  • Preventative consulting for avoiding the risk of criminal liability
  • Assistance in the course of search warrants conducted by customs, tax office investigators, and prosecutors
  • Representation in criminal investigative proceedings
  • Representation before social insurance authorities
  • Representation in judicial criminal proceedings
  • Individual consultations on social insurance law issues, e.g. preparation of expert opinions
  • Support of colleagues (with a client protection agreement)
Ulrich Derlien
  • Partner | Lawyer, Tax consultant

employee profit participation.

We develop and optimize management and employee participation models on the backdrop of government subsidies and the positive impacts and incentives within the company.
corporate law implementation.
tax audits.
carry models.

corporate law implementation.

Employee participation as a tool for attracting, committing, and motivating valuable talent.


  • Structuring an appropriate employee participation model
  • Development of virtual participation models with various participation options
  • Accepting employees into the shareholder structure or granting share options
  • Support in the event of an employee's departure from the company and its participation model
  • Support in the ongoing management of the participation model
  • Establishing the corporate law prerequisites for granting shares to employees

Ulrich Derlien
  • Partner | Lawyer, Tax consultant

posting employees.

Expert teams assembled on an individual and multidisciplinary basis advise and provide support to our clients with comprehensive expertise in all aspects of posting employees — nationally and internationally.

subject areas.

  • Tax aspects for an international staff assignment
  • Social insurance law for an international staff assignment
  • Employer's social insurance obligations in the case of a foreign assignment


  • Consulting for the posting company and the posted employees
  • Preparing and revising labor law rules on posting employees
  • Generating payroll accounting
  • Preparing tax returns
  • Contract drafting
  • Employing staff across borders
  • Assessment of tax liabilities
  • Evaluation of tax questions for the international employment of staff
  • Consideration of the applicable social insurance law
  • Application for A1 certificates and confirmation of expatriate employee status
  • Evaluation of social insurance law contribution obligations abroad, also in cooperation with foreign network partners
  • Assessment of special questions on occupational safety abroad (such as working hours regulations), and consulting on measures for worker protection, accident prevention, and representation in the event of recourse asserted by workers’ compensation insurance carriers
Prof. Dr. Andreas Katzer
  • Partner | Lawyer, M.I.L (Lund)

Wir verzahnen wirtschaftliche, steuerliche und rechtliche Aspekte und beraten fachübergreifend und auf Augenhöhe.

Wir begleiten große Familienvermögen, um langfristige Werte über Generationen zu bewahren.

Wir bieten maßgeschneiderte, praktikable Lösungen zu IT-Compliance, Digitalisierung und IT-Projektmanagement.

Wir beraten, bewerten und prüfen – national, aber auch international als unabhängiger MOORE-Netzwerkpartner.