All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
Designing a website which advertises properties is permissible in principle and taking money on this work is lawful.
As regards the prohibition which you mentioned in the question, it does not prevent from designing the website. The website designer is not sinful if some people who benefit from his services buy the houses and properties through a Riba based loan (interest and/or usury) and he is not considered as helping them in Riba or causing it as long as this website is not advertising for banks which deal with Riba or for Riba based transactions.