You have hosting for one project and a second domain comes along. It could be a new site for another venture, a client's site if you freelance, or simply an idea you want to try without paying for another account. The question is whether you need to buy hosting again or whether the plan you already have is enough.
In most cases it is enough, and the tool that makes it possible is called an addon domain.
What an addon domain is
An addon domain, in cPanel terminology, is a second full domain you attach to a hosting account you already own. It is not a subdomain of the first one: it is its own domain, with its own address, living inside the same account and sharing the resources of the same plan.
Each addon domain gets its own folder, its own database if needed, and is administered independently of the main site. For a visitor there is no difference at all between a site hosted as the primary domain and one hosted as an addon: the address is just as real, the site loads the same way, and Google indexes it the same way.
Addon domain vs. subdomain: not the same thing
These two get mixed up often. A subdomain (like blog.yoursite.com) is still part of the same domain: it shares the brand and, for search engines, stays associated with the main domain. An addon domain is a completely separate address (like anotherproject.com), with no visible relationship to the first.
The choice depends on what you are building. If the second site extends the first (a shop, a blog, an app), a subdomain makes more sense because it inherits part of the main domain's reputation. If it is an independent project with its own brand, an addon domain is the right call: a client's site has no business hanging off your own domain.
How to add one, step by step
From the control panel, find the addon domains section. The process asks for three things: the name of the new domain, the folder where it will live inside the account, and it confirms the creation of an associated technical subdomain (which in practice you will not use once the domain is pointed correctly).
Before adding it you need to have pointed that domain's name servers at your hosting, the same as with any new domain. If the domain was just registered, it can take a few hours to propagate before the new site loads at its own address.
Once created, install whatever you need inside that folder: WordPress from the application installer, or the files by hand if you are bringing them from elsewhere. It is an independent site from the first minute.
Resources are shared, and that has a ceiling
This is the part worth understanding clearly before adding domains without thinking it through. Every site on the account (the primary one and every addon) shares the same processor, memory and database allowance of the plan you bought. If one domain gets a traffic spike, that consumption takes capacity away from the others.
For two or three small sites with normal traffic, this goes unnoticed. The problem shows up once one of the sites starts growing seriously: it then competes for the same resources as the rest, and everyone feels it during the busiest moments.
The sign that it is time to split: if one domain on the account starts running slow during another one's traffic peaks, or if one of them grows to a volume that justifies its own plan.
When to split into separate accounts
It makes sense to split in three concrete situations. When one of the sites belongs to a client and the relationship might end: if they ever leave, it is far simpler to hand over or cancel a standalone account than to unwind an addon domain from yours. When a site starts consuming resources disproportionately compared with the others. And when you need to isolate security: if one domain gets infected with malware, sharing an account raises the risk of it spreading into the other files compared with a separate account.
For anyone administering several client sites professionally, the tidier alternative to piling up addon domains is moving to the reseller model, which gives each client an independent account without paying for full hosting per client.
The real advantages of using them well
For a developer or agency with several small projects, a plan allowing multiple domains is cheaper than buying separate hosting for each one. For someone with a venture and a parallel idea, it is the way to test a second project without committing to new spending. And administratively, having everything in one panel simplifies things: one place to check backups, security certificates and access, instead of scattered across several accounts.
How to keep several domains organised
With two or three sites on the same account, getting organised from day one saves confusion later. Give each addon domain a folder name you can identify at a glance, instead of leaving whatever the panel suggests by default: six months from now, "folder3" tells you nothing, while a name tied to the project does.
Also create a separate mailbox for each domain, even a fresh one, instead of reusing the main domain's inbox for every site's notices. When a notification arrives (a renewal, a certificate about to expire) it is immediately clear which project it belongs to.
And keep a simple record, even a spreadsheet, of which domain lives in which folder, with which database, and since when. It looks unnecessary at first, but it is exactly what saves you time the day you need to migrate just one of them or hand it over to a client.