Buying or living in a new build house does not mean that you cannot extend the property. Often when planning permission is granted for a new build house, then the permitted development rights are removed from the property. This means that any extension would need to be applied for under a house holder planning application. In some areas, this can limit or restrict what you can do. If you live in an area liable to flooding, sometimes the only way you can extend is under permitted development. If the house is on the green belt, then the house builder that built the property could have already extended the house to the limits allowed. I would certainly recommend first checking if you have permitted development rights and seeing if you are in a flood zone or within the green belt.
It does not mean you cannot extend; we have worked with many clients that live in a new build property and have gained permission for an extension or loft conversion. It may just mean you have to pay slightly more attention to where you extend and how large the extension is than you would have to if you have permitted development rights on the property.