you must tell them. The rate they use could be higher than a private house if the underwriter feels the risk of misfortune is higher. If you were mixing explosive substances, the fire/explosion rate woudl certainly be higher. You may have customers/suppliers calling to the house, thus increasing the public liability risk. You may have employees, even casual ones, calling, therefore you might need extra employers liability cover.
Alternatively, you might have a very risk averse business, like addressing envelopes, with no employees and you drop off and collect all materials, which would probably result in no extra premium.
On the contents side, you might want cover for your busines contents. is the phone/fax/PC a business item or a personal item??? hard to tell if you are the insurance company dealing with the claim.