Work on fixed water and heating installations needs an autoriseret VVS-installatør (authorised plumbing contractor), or a firm that brings one. Ask which it is, and ask for that on the invoice (faktura). Authorisations are looked up at Sikkerhedsstyrelsen, sik.dk.
Roskilde covers a tight centre and the area around it, including Skibby. Many houses are older and have a garden. Access around the back, across gravel, is common. The same 4 businesses as the Danish page: AJ VVS og Teknik A/S, ERSA VVS, Højer VVS, THORSEN OG PETERSEN VVS A/S.