Patent of addition

You improved your own patented invention. The improvement is not inventive over your own patent. There is a specific mechanism for exactly this.

In simple words

An application for an improvement or modification of an invention for which you already have a patent or a pending application, granted for the unexpired term of the main patent and without separate renewal fees.

Think of it as

An extension wing on a building you already own.

Why it matters

Its advantage is that the main patent cannot be used to attack the addition for lack of inventive step. Its cost is that it lives and dies with the main patent - if the main patent ceases, so does the addition, unless it is converted to an independent patent.

A simple example

You hold a patent on a filter. A year later you find a better housing geometry that would be obvious over your own filter. A patent of addition may be the right route rather than a fresh application.

Common mistake

Using a patent of addition for something that is genuinely inventive on its own. That gives away independent term and independent life for no reason.

Where it comes from

Patents Act 1970, sections 54, 55 and 56