Most cracks in most houses do not mean the foundation is failing. Plaster cracks, settlement cracks in new builds, thermal cracks in long terraces, the great majority are cosmetic, and a homeowner who panics at every hairline ends up spending money on problems that do not exist. The skill is recognising the small minority of signs that deserve a phone call.
Cracks that matter
- Wider than a one-pound coin (about 3mm).
- Diagonal, running from a window or door corner.
- Stepping through mortar joints rather than splitting bricks.
- Visible from both inside and outside on the same wall.
- Growing visibly week on week.
Cracks that usually do not matter
- Hairline cracks in plaster that follow ceiling-wall junctions.
- Vertical cracks at the join between an extension and the original house.
- Settlement cracks in a property less than two years old.
- Cracks that have been there for decades and never grown.
Other signals worth attention
Doors and windows that have suddenly started to bind on the same side of the house. A bay window that has tilted. Gaps opening between the main house and a single-storey extension. New cracks appearing during or shortly after a hot summer, or after a leaking drain has been discovered nearby. Any of these alongside cracking in the same area shifts the picture from cosmetic to structural.
What to do first
Before getting any quotes for repair, get a structural engineer's opinion. A short visit and a written report typically costs £400 to £900 and tells you whether you have a problem, what is causing it, and what category of repair is appropriate. That report then drives the contractor quotes, without it, you are inviting wildly different scopes and prices. Our process page walks through how that fits into the wider job, and how long underpinning takes sets realistic expectations on timing.
What not to do
Do not fill cracks with decorator's caulk before the engineer has seen them. Do not get cracks redecorated. Do not accept a contractor quote that has not been preceded by a written diagnosis. And do not assume your buildings insurer will refuse the claim, subsidence cover is standard on most UK home policies and the claim is worth making.