Need advice on how to transfer a domain to squarespace

My organisation is planning to transfer our website to squarespace. I have read through the pages about transferring the domain, but I have a few questions. We were hoping to create the new website on squarespace and once it was ready, transfer the domain from the old website. Will we have to build the new website on squarespace during the 14 day free trial? The old website is still active and using our domain, and we want to leave it that way until we are ready to switch so that the website isn't down for weeks at a time. Does anyone have any advice for the best way to handle this switch?

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Will we have to build the new website on squarespace during the 14 day free trial?

You can build during the trial and beyond. Just buy the plan you will need for your needs at the end of your trial. You can take as long as you like to build your new site. In other words paying for a site does not mean you also have to transfer your domain at the same time. Transferring the domain can take place at any time, after you have paid for your site.