From a sustainability perspective it is essential to minimize the consumption of potable water in buildings. One approach is to use alternative water sources for applications that do not require potable water. These applications include toilet flushing, indoor and outdoor cleaning, laundry and garden watering. The use of rainwater collected from roofs for these types of applications is common practice in Belgium. However, because of longer and more frequent periods of drought in recent years, we are increasingly confronted with a lack of available reusable rainwater. Additionally, in buildings with a limited roof surface area relative to the number of users (e.g. hotels), the available rainwater is insufficient to cover the need. Therefore, also other water sources are considered such as greywater, whose availability remains unaffected by climate change or roof surface area. Greywater is wastewater from sanitary building equipment, excluding toilets and urinals. It is a daily source of water whose volume is generally well in line with what is needed for non-sanitary uses. Meanwhile, various domestic greywater treatment systems, designed to prepare water for reuse, have become available on the market. Yet, little is known about the quality of the treated greywater they deliver for reuse. To address this, we conducted a sampling campaign on installed greywater treatment systems including biomechanical and biological plant-based systems. The latter concerns a façade greening system (Living Wall System) for greywater treatment. Samples from both raw and treated greywater were collected, along with rainwater samples as reference. The samples were analysed for various physicochemical, microbiological, and wastewater quality parameters. Our findings, discussed in this paper, confirm the need for greywater treatment before reuse. Furthermore, the treatment systems we studied produced greywater of a quality comparable to that of commonly used untreated rainwater, opening up new possibilities for employing treated greywater in applications traditionally reserved for rainwater. But, spray applications are not recommended, as the bacterial levels in both treated greywater and rainwater samples exceed the guideline values set by standard EN 16941-2.