This page is only designed to give you an overview of the local requirements, but it is your duty to know and comply with the local laws. misterb&b is not a legal adviser : you should read carefully the relevant laws and contact a local adviser or the local authorities for further understanding.
The Statutes on the Protection and Preservation of Housing in Cologne (Wohnraumschutzsatzung) of 4 July 2014 regulates the short-term rentals in the City. The Statutes indicates the cased constituting misappropriation/misuse of residential housing (specially article 4). For instance, turning more than 50% of the floor for commercial purpose is considered a misappropriation and can only be allowed by applying for an authorization. Visit this page about housing on the official website of the City of Cologne for more information.
Housings must respect some minimum requirements and standards : check out the local regulations, in particular the Housing Supervision Act (Wohnungsaufsichtsgesetz -WAG NRW) of April 10, 2014 (especially article 4).
To check the legality of your short-term rental, contact the Office for Housing of the City of Cologne.
The Statutes on the Protection and Preservation of Housing in Cologne (Wohnraumschutzsatzung) requires to obtain an authorization for misappropriation for the use of housing other than residential. The authorization procedure may demand to substitute the space not dedicated to residential housing by creating equivalent space for residency in the City. See this page and this brochure to learn more about the procedure : you can find here the application form for misappropriation.
Read this brochure for explanation about misappropriation and application for authorization.
Tourist tax (Kultur- und Tourismustaxe)
The City of Cologne levies a cultural subsidy tax on overnight stays in private accommodations. The tax rate is 5 % of the amount paid by Guests for the accommodation, including value added tax (however excluding the tax for the promotion and advancement of culture). You can find all informations about collection and remittance of the tax on this page of the official Cologne website. Moreover, this FAQ may turn out to be relevant to you.