Where two properties share access with a registered easement or agreement through Information Services Corporation setting out each party's rights and responsibilities, buyers generally treat it as a minor inconvenience rather than a major flaw, since the terms are settled and enforceable.
Where there is no registered agreement, just a longstanding handshake between the current neighbours, the discount is larger because a lawyer reviewing the file for a buyer will flag it as an open liability. That uncertainty tends to worry buyers more than the physical inconvenience of sharing the pavement.
Fixing the paperwork before listing, by having a lawyer draft and register a proper agreement through Information Services Corporation, usually costs within the $800 to $1,500 range that legal work on a residential file runs, and it often recovers more value than it costs. Get your hand-reviewed valuation from Joel Dyck.