What you add to a letterhead is completely up to you! However, there are some elements that a professional letterhead should include: company name, logo, and contact details. You can also add images, graphics, slogans, links to social media pages, and other elements to your letterheads.
When it comes to legal requirements, your letterhead should include:
The name of your business
Where your business is registered
The registered company number
The address of the registered office