The short answer
Red flags include flats above commercial premises, mixed-use buildings, non-standard construction, high-rise properties, fire-safety concerns, and high flood-risk locations.
None of these automatically make a property unmortgageable, but each one shrinks the list of lenders willing to consider it. Knowing which of these apply before you fall in love with a property saves a lot of wasted applications.
Why this matters for your purchase
The building safety section of a PropM8 report is built around the exact red flags lenders look for, so you know before you fall in love with a property whether it's likely to be an easy mortgage or a difficult one.