Measure more than the opening
Request width, depth, height, access route, surrounding connected air space, and any obstruction. Identify where the intake and exhaust would be, with the room and access panels in their normal finished condition.
Ask whether the plan uses the niche's air, a connected room, or manufacturer-approved ducting. Have the provider show the approved arrangement and the reason it meets the selected model's requirements. An open doorway during installation is not a permanent design.
Check the whole location
Have the installer assess winter temperatures, support and level placement, moisture, drainage, piping, electrical access, and the ability to remove service panels. Describe dirt, workshop dust, stored chemicals, or other nearby uses.
If building or crawl-space correction is needed, obtain appropriate professional evaluation and a written scope. Do not cut openings, route air through another space, or add an improvised fan to make the heater fit.
Include future service
Ask how a technician would reach the fan, controls, tank connections, drain, and other service parts. Request the path for eventual replacement without assuming walls or equipment can simply be moved later.
Compare the niche plan with another location, including related trade work and hot-water delivery. A less expensive equipment placement may be less attractive once its site corrections and access needs are included.
Your next step is to obtain a measured layout and a model-specific fit explanation. If the provider cannot show a supported air path and safe service access, choose a different plan before buying.