Once a plumbing and heating firm has engineers on the road and years of trading behind it, the website stops being a listing and starts being a reference. Homeowners comparing quotes for a boiler or a full heating system check it before they commit. Landlords and letting agents weighing up who gets the annual servicing across a portfolio check it. Referrals given your name by a neighbour check it before they ring.
Every one of them is really asking the same question: is this an established firm, or one van and a mobile number? Gas work makes people careful. A dated template site with the Gas Safe logo pasted in a corner answers that question badly, however good the firm behind it is.
Plumber website design at this level means building the reputation infrastructure behind the firm: registrations, memberships, coverage, the contract side of the business, all presented at the standard the work is done to. It decides which work you are considered for.