You absolutely can build a Squarespace website yourself. I would be suspicious of any designer who pretended otherwise. The platform gives you templates, hosting and a visual editor precisely so a normal business owner can create and manage a website.
That does not mean everybody should do it. The work is rarely just dragging blocks around. You still need a clear offer, useful content, sensible pages, good images and enough patience to make the mobile version work properly.
When Building It Yourself Makes Sense
DIY is a good option when money is tighter than time, the website is simple and you are comfortable learning as you go. It also suits somebody who enjoys design, already has strong written content and does not mind solving the odd layout problem.
You will learn how the site works, retain complete control and can spend the design fee elsewhere in the business. For a simple early-stage idea, that may be exactly the right trade-off.
Where DIY Websites Usually Get Stuck
The template is often the easy bit. Deciding what belongs on the homepage, writing about the business, choosing useful images and keeping the navigation simple can take much longer. Mobile layouts, ecommerce settings, domains and search-facing page details also tend to arrive at the end when enthusiasm is running low.
None of that is impossible. It is simply work, and it competes with the work you already do for customers.
What a Freelance Squarespace Designer Adds
A good freelancer should help you make decisions, organise the pages and turn supplied material into a website customers can understand. You should know who is building the site and be able to speak to that person directly.
I also use custom code where it provides a useful improvement, but most small businesses do not need a heavily bespoke system. The point is to choose the simplest approach that does the job properly.
What You Still Need to Provide
Hiring a designer does not remove the need for your knowledge. You know the business, customers, services and common questions better than anybody. I can help shape and organise that material, but I do not currently sell copywriting or invent claims on a client's behalf.
Good photos, timely feedback and somebody available to approve decisions make any project run more smoothly.
How to Choose Without Regretting It
Estimate honestly how many hours you can give the project and whether there is a fixed date by which it needs to be useful. Then compare that with the cost of help. Ask any designer what is included, who owns the account, what you pay after launch and whether you can edit the site yourself.
If you want direct help, see my Squarespace website design service and published prices. If DIY still looks sensible after that, go for it.
Conclusion
Build it yourself when you have the time, interest and a simple enough brief. Hire a freelancer when getting the structure right and getting the site launched are worth more than the design fee. Choose an agency when the project genuinely needs a team.
There is no universally correct answer. The sensible choice is the one that fits the business now without locking you into costs or complexity you do not need.