A common question I get is "Can I put more than one invention into my patent application?". The short answer is "yes". The more complex answer is "yes - but the patent examiner may ask you to split them out later". When you file your patent application you have to pay the patent office a "filing fee", a "search fee" and an "examination fee". The total of all three of these fees is currently $400 for most inventors who can qualify for a micro entity discount. If you have more than one invention in your application, the examiner will likely ask you to split them out because remember you only paid them one fee for one search and one examination. Each invention (if they are truly different) will need to have it's own search and examination done. Now, there are some cases where you can try to argue that the inventions are similar enough that the examiner should be able to cover them all with just one search and one exam but in my experience these arguments are usually ignored (at best) or come back to haunt you in the future (at worst).
Why squeeze multiple inventions into just one patent filing?
There are many reasons why people want to add multiple inventions into a single patent application. The first one is cost savings. When I first started of filing patents back in 2000, our client was a large biotech company trying to patent 1000s of new genes and proteins all at once. At $700+ in government filing fees they did not have that money to file a separate patent application for each new gene discovery (this would have costs them millions in filing fees) so our strategy was to combine as many discoveries as we could into a single application and pay just one fee. We later could go back and split them out as needed (or wait for the examiner to force us to split them out). Another more common reason is that sometimes there are different versions or different designs of your invention and you don't want to split them out because you think they are all related and have the same functions and goals. In these cases it may make sense to try and group them together into just one application. When we write the claims, we will try to cover the most important pieces or elements shared by all the designs giving the application "unity" and hopefully getting the examiner to examine them all together.
Downside to combining inventions together?
There a few risks and drawbacks to combining multiple inventions into the same patent. The first one that comes to mind is something called "patent term". A patent will be active and enforceable for up to 20 years from the date you filed it. If you have multiple inventions all combined together, they may all be clustered together into the same patent term calculation meaning they will all expire at the same time. If you are making millions (or billions) per year on your invention like some drug companies do, you try very hard to figure out how to extend your patent term or stagger the expiration of your patents (not have them all expire at the same time). Another potential drawback is asset reduction. What I mean by this is that patents and patent applications can be a very valuable asset to a company. If you have just one patent or patent application some investors or potential business partners may not value that as much as if you had multiple patents or patent applications. A new start-up company could potentially get more funding if they had multiple patents surrounding their inventions vs. just having filed a single patent application.