Most of the time people have several ideas in their mind for the kind of home they wish to build. But those vivid ideas are nowhere near to what an architect can do for them. People might think they don't need an architect, but if they try to understand the service he can provide, their view shall change.
In case of minor renovations or extensions, one might wish to skip the idea of hiring an architect. But the fact is that for any kind of modifications, you do need a design professional who can provide you with an exact drawing of your project. They can guide you throughout the process by designing your space according to your desire and budget.
An architect has several connections with the best builders, interior designers, and other service personnel. These connections can help you get the best people at your service. This is why it's very important to hire an architect for home extensions. There are also extension architects who specifically work on the extension projects. They are the right people to consult for creating the best in the available space.
Why can't we just depend on the builder for design?
The one-line answer is- because it's not their job! You have to understand that an architect is a bridge between you and the builder. The architect understands your desires and vision and creates it on paper with his drawings and specifications. These drawings are a map for the builder. There is no point in contacting the builder and discussing the cost with him without showing the plan. Therefore, it's necessary to hire an architect who can draw the blueprint for you and later communicate with the builder for finalizing the cost. This is the right way and saves you a lot of time and money.
Can we substitute an architect with a builder that provides an in-house design service?
The builders nowadays have started providing in-house design services to help the clients have a one-stop destination for all their needs. Such projects are called "design and build". These services can be beneficial sometimes but to be honest, you must first interact with the builder's design professional. Communicate with him/her about your needs and if you think they can connect with your vision, you can go for them. But if you are not satisfied, it's best to hire an extension architect for your home extension project.