Work on fixed installations (faste installationer) has to be done by an autoriseret elinstallatørvirksomhed (authorised electrical firm). Ask whether they hold the authorisation themselves or bring someone who does, and get that on the invoice.
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 Sikring og Teknik ApS, GORDON-EL, NBB El ApS, Wibholm Aut. El-installatør.