Software house in practice
A software house is a company whose trade is writing software. The result of its work is not a campaign or a flyer, but a working program: management software, a web app, a smartphone app, a platform. Some develop only on commission, that is custom software for clients; others create their own products and sell them on subscription (SaaS); many do both.
The difference from a web agency or a communication agency lies in the core of the work: the agency deals with websites, content and advertising; the software house builds applications, with everything that entails — process analysis, databases, connections through APIs, testing and maintenance. Hybrids exist too: DT Digital, for example, is a software house that also takes care of websites, SEO and communication, so the client has a single partner.
You need a software house when an off-the-shelf program is not enough: your way of working is specific, the programs you use don't talk to each other, or you want to turn an idea into a digital product of your own. In these cases buying licences doesn't solve it: you need someone who analyses the processes and builds the right tool.
How to choose one: look at past projects, ask who owns the code and the data (they must be yours), how maintenance works after delivery and, if you can, talk directly with the people who will build it. Be wary of anyone proposing the solution before understanding how you work.