A venue finder could let people search for the features they need, such as step-free access, accessible toilets or quieter visiting periods. Specific information is more useful than a single universal accessibility rating.
Each detail should identify its source and review date. Venue changes and user corrections need a moderation process. Photographs or measurements can help where supplied appropriately, but the app should explain gaps instead of guessing.
Start with a manageable local area and involve people with varied access requirements in testing. Include a temporarily unavailable feature and a venue whose information is incomplete. The app can help users ask better questions and compare options, while the venue remains responsible for confirming the arrangements available for a particular visit.
Related reading: Mobile App Accessibility: Make Your App Easier to Use.