Combining design and marketing in one supplier is efficient when the site exists to generate enquiries, because the people measuring the result can change the thing producing it. It goes wrong when the combination becomes a retainer with no measurable objective, which is a common and comfortable place for both parties to end up.
The genuine argument for one supplier
When a site's purpose is enquiries or sales, design and marketing are the same problem seen from two ends. A combined supplier can test a headline, change a form, and see the effect without a handover or a blame conversation. That closed loop is worth real money, and it is the actual case for buying both together rather than the convenience of one invoice, which is what is usually offered as the reason.
Where web design marketing and website development marketing retainers go wrong
They drift into activity without objective: a monthly report of work done, posts published and minor edits made, with no agreed measure of whether anything improved. Prevent it by agreeing the measure before the contract, enquiries of a defined quality, cost per enquiry, or revenue, and reviewing against it on a fixed cadence with the explicit option to stop. A supplier confident in its work will accept that; one that resists is telling you something.
Insist on owning the accounts and the data
Analytics, advertising accounts, search console, the domain and the site itself should be in your name with the agency granted access. This is the single most common source of pain when a relationship ends, and it is also the thing that makes performance claims checkable while it runs. An agency that prefers to hold accounts on your behalf should be asked why, and the answer is unlikely to be for your benefit.
Be sceptical of guarantees, and read the published guidance
Google's own guidance on hiring an SEO is worth reading before you sign anything, particularly its position on guaranteed rankings. No supplier controls the ranking system, so a guarantee is either meaningless or a promise about terms nobody searches. Ask instead what the supplier will change, how it will measure whether it worked, and what it would do if the measurement says it did not.
Questions people ask about web design and digital marketing
Should one agency do both our website design and digital marketing?
It is genuinely efficient when the site exists to generate enquiries, because the people measuring the result can change the thing producing it. The case is that closed loop, not the convenience of a single invoice.
How do we stop a combined retainer drifting?
Agree the measure before signing, enquiries of defined quality, cost per enquiry or revenue, review on a fixed cadence, and keep an explicit option to stop. A confident supplier accepts that arrangement.
What about guaranteed rankings?
No supplier controls the ranking system, so a guarantee is either meaningless or a promise about terms nobody searches. Google's guidance on hiring an SEO addresses this directly and is worth reading before signing.