The simple principle to follow is always to be truthful and clear. Also, never oversell yourself or be economical with the truth. This is a basic tenet of the RIBA and Arb codes of professional conduct.
It is quite legitimate in your new practice brochure and website, or your personal CV and portfolio, to explain and illustrate your previous work as long as you acknowledge the practice you were working for and the contribution you made to that work.
The intellectual property right and copyright of the work you do as an employee is likely to belong to your employer unless it is specifically stated otherwise in your employment contract. Any drawings, photographs or written information will belong to the practice you worked for.
Therefore you should seek the written permission of that practice to use any of their material.
Sometimes the client will have obtained the copyright and even the intellectual property right for the design and any material on their project. If this is the case, then your previous practice would have to get their client’s permission for you to use it.
There is nothing to stop you using your own photographs of your previous projects, if that view is in the public domain. This includes photos of external elevations viewable from the street of publicly accessible areas. Photos of private areas not available to the public would require the permission of the client, and possibly the occupant.
When you start employing your first staff in your new practice, it is worth addressing this issue in their employment contracts. Guidance is available in the RIBA Good Practice Guide on Employment.
So for any information from your previous work for other practices that you wish to use to promote your own practice, or even in your CV for a new employer, you should obtain written permission from the relevant previous employer.
It is also sensible to get their approval for how you intend to acknowledge them in the information such as: “This is the Stirling Prize-winning building. I designed the door handles while working as a project architect for Guru Architects. It also won the best designed ironmongery award from the Guild of Architectural Ironmongers.”