Most people who want an agency start by describing everything they can do. It reads well and sells badly, because a prospective client cannot tell what they are buying.
A narrow service is easier to price, easier to explain and much easier to repeat.
What a narrow service looks like
Not "web design" but "a one page site for a trade business, delivered in five days". Not "marketing" but "a monthly newsletter written from your existing material".
The narrower version can be quoted without a meeting, delivered on a checklist and improved every time you repeat it.
The arithmetic of the first ten clients
Suppose the service is priced at 250 euro and takes eight hours end to end, including the client conversation and revisions. That is roughly 31 euro an hour, before finding the client.
Finding the client is the real cost early on. Ten to twenty hours of outreach per signed client is normal at the start, which drops the effective rate to around 12 euro an hour for the first few.
By client five or six, delivery time usually falls to five or six hours and referrals begin to reduce outreach. The effective rate roughly doubles without changing the price.
Why this beats starting as an agency
An agency has fixed costs before revenue: a brand, a site, a service menu, possibly contractors. A narrow service has almost none. You can test whether anyone will pay within a few weeks.
An agency also requires you to price work you have never done. A repeated service teaches you your real delivery time, which is the only sound basis for a price.
When to raise the price
Raise the price when you can deliver the current version reliably and you have turned down work. Not before. A price rise with no queue simply reduces enquiries.
Going from 250 to 400 euro on the same eight hour service is usually possible by the tenth client, because by then you have examples, a process and references.
When an agency actually makes sense
When one service is consistently full, the delivery is documented well enough that another person can follow it, and clients are asking for adjacent work. That is a real signal.
Building the structure first, in the hope the demand follows, is where most of the money and enthusiasm goes.
The honest risks
Service income stops when you stop. Clients leave. A narrow service can be undercut. None of this is avoidable, and it is why many people eventually move part of their time into products.
Starting narrow does not remove those risks. It just finds out sooner, and much more cheaply, whether the work sells at all.