The company, which now manages more than 70 apartments, opened its first property in 1997, with the latest units opening in December 2017.
The occasion was celebrated with a "birthday bash" for guests, friends, suppliers and staff in the garden of Cotels' HQ, located at the 7Zero1 development in Milton Keynes. 7Zero1 is Cotels flagship property, with a mixture of one- and two-bedroom units.
Marcia Gomez, managing director of Cotels, said: "We were thrilled to welcome everyone to our event and we could not have wished for a better day with the wonderful sunshine. This is a huge milestone for Cotels. When we opened the first apartment we could have hardly envisaged that we would be here 21 years later having looked after thousands of visitors to Milton Keynes, Northampton and Luton."
Cotels is a sponsor of the 2018 Serviced Apartment Summit Europe which takes place in London next week.