BACKGROUND: Over the last decades, many high-income countries have successfully implemented assertive outreach mental health services for acute care. Despite evidence that these services entail several benefits for service users, Germany has lagged behind and has been slow in implementing outreach services. In 2018, a new law enabled national mental health care providers to implement team-based crisis intervention services on a regular basis, allowing for different forms of Inpatient Equivalent Home Treatment (IEHT). IEHT is similar to the internationally known Home Treatment or Crisis Resolution Teams. It provides acute psychiatric treatment at the user's home, similar to inpatient hospital treatment in terms of content, flexibility, and complexity. METHODS/DESIGN: The presented naturalistic, quasi-experimental cohort study will evaluate IEHT in ten hospitals running IEHT services in different German regions. Within a multi-method research approach, it will evaluate stakeholders' experiences of care, service use, efficacy, costs, treatment processes and implementation processes of IEHT from different perspectives. Quantitative surveys will be used to recruit 360 service users. Subsequently, 180 service users receiving IEHT will be compared with 180 matched statistical 'twins' receiving standard inpatient treatment. Assessments will take place at baseline as well as after 6 and 12 months. The primary outcome is the hospital re-admission rate within 12 months. Secondary outcomes include the combined readmission rate, total number of inpatient hospital days, treatment discontinuation rate, quality of life, psycho-social functioning, job integration, recovery, satisfaction with care, shared decision-making, and treatment costs. Additionally, the study will assess the burden of care and satisfaction with care among relatives or informal caregivers. A collaborative research team made up of researchers with and without lived experience of mental distress will conduct qualitative investigations with service users, caregivers and IEHT staff teams to explore critical ingredients and interactions between implementation processes, treatment processes, and outcomes from a stakeholder perspective. DISCUSSION: By integrating outcome, process and implementation research as well as different stakeholder perspectives and experiences in one study, this trial captures the various facets of IEHT as a special form of home treatment. Therefore, it allows for an adequate, comprehensive evaluation on different levels of this complex intervention. TRIAL REGISTRATION: Trial registrations: 1) German Clinical Trials Register (DRKS), DRKS000224769. Registered December 3rd 2020, https://www.drks.de/drks_web/setLocale_EN.do ; 2) ClinicalTrials.gov, Identifier: NCT0474550 . Registered February 9th 2021.