Most people who work from home will in all likelihood have only two complaints about their home set up one being space constraints and the other lack of privacy.
Garden offices are available in modular forms through construction companies that specialize in the same or can be constructed by users to their specific needs. Most often these buildings have a toughened glass front façade to provide a more professional look.
Essentially all material to build a garden office is available at most hardware stores. Make a plan or consult an architect for the same. Ensure that you have proper insulation at the floor and roof levels and good quality sealants and cladding on the inner and outer walls to protect from the elements.
Allow for any plumbing and most electric requirements at the beginning itself. Finally have all windows, doors and other openings secured with sturdy locks or use an electronic locking system for added safety.