The first few weeks
Very little that is visible. The changes in this period are chemical and biological rather than structural.
Plumbing traps begin evaporating. In dry winter air with the heating running, an unused fixture can lose its seal within two to three weeks, at which point sewer gas enters the building. This is the single most common reason a house that has been empty for a month smells wrong, and it is entirely reversible by running the tap.
Air becomes stale without the ventilation ordinary occupancy provides. Dust settles undisturbed. Refrigerated food begins to be a problem if it was left.
One to three months
Pests are the main development, particularly through autumn. A quiet building with no vibration and no human scent is preferable to an occupied one, and a mouse needs an opening of about six millimetres.
Any slow leak that started is now doing real damage, because it has been running the whole time. This is the period in which the difference between a monitored and unmonitored property becomes financially significant.
Externally, the property begins signalling vacancy: grass, flyers, an unchanged appearance, snow that nobody clears.
Three months to a year
Moisture problems compound. Any water intrusion that has occurred has had time to saturate materials, and mould develops in unventilated spaces.
Rodent populations establish rather than merely visit, with damage to insulation, wiring and stored materials.
Exterior maintenance issues that would have been caught become consequential — a lifted shingle becomes a leak, a blocked gutter becomes a foundation problem, a small drainage issue becomes a wet basement.
Systems left idle may not restart cleanly. Pumps seize. Seals dry. Batteries in alarms and detectors fail.
Beyond a year
The property requires meaningful work to return to occupancy rather than simply cleaning. Mechanical systems generally need professional attention before being relied upon. Any moisture problem is now a remediation project.
Insurance is usually the more pressing constraint by this point — most standard policies will have moved the property into restriction or required a specialised vacant property policy well before a year.
What prevents all of it
Almost everything on this page is prevented by ordinary attention rather than by anything sophisticated. Heat maintained or water removed. Fixtures run. Entry points sealed. The property visited on a schedule so that problems are found while they are still small.
It is unattended vacancy that damages houses, not vacancy. A property that is empty but looked after can sit for years without meaningful deterioration.
Related questions
How long can a house sit empty?
Physically, a house can sit empty indefinitely if it is maintained — heated, water managed, monitored and looked after. The limiting factors are not structural, they are practical: insurance, deterioration and the accumulation of unnoticed problems.
The insurance constraint is usually the binding one. Many policies begin restricting coverage after roughly thirty days of vacancy, though the threshold and the consequences vary. Beyond that point some owners need a specialised vacant-property policy, which is a different product with different terms and typically a higher premium.
The deterioration constraint is gradual and cumulative. Traps dry within weeks. Pests establish over a season. Moisture problems compound. Systems that sit idle behave unpredictably when restarted. None of these are dramatic on their own; together over a year they produce a property that needs significant work.
The real answer is that a house can sit empty for a long time safely, provided somebody is attending to it. It is unattended vacancy rather than vacancy itself that causes the damage.
What actually happens to a house when it sits empty?
Less dramatically than people imagine in the first month, and more than they expect over a year.
Within weeks: plumbing traps dry out and sewer gas enters the building, producing the characteristic stale smell of an empty house. Dust settles undisturbed. Air becomes stale without the ventilation that ordinary occupancy provides.
Within months: pests find their way in, particularly through the fall. Small leaks that would have been noticed immediately run continuously. Moisture accumulates in unventilated spaces. Landscaping deteriorates visibly, and the property starts announcing its own vacancy.
Over a year or more: systems left idle can seize or fail on restart. Moisture problems develop into mould. Rodent populations establish. Exterior maintenance issues compound — a small roof problem becomes a leak, a drainage issue becomes a basement problem.
Almost all of this is prevented by ordinary attention: heat maintained, water managed, fixtures run, the property visited, and problems caught while they are still small.