Subsidence, settlement and heave
What should you make of cracks in an extension that is only two years old and stands separate from the main house?
Read them first as shrinkage and early bedding-in of new work, not as a label on the old house. New walls, new plaster and a new junction dry and shorten. The survey can record the age you were told and the cracks it saw. It cannot say they have finished, because one visit does not monitor a crack and does not test the soil.
Last reviewed 24 October 2026. Written for buyers in England and Wales unless this answer says otherwise. No named surveyor has signed this page. It is general guidance, not a report on your house, and not legal advice.
What is the fuller explanation?
A two-year-old extension has its own footing, its own roof and often a different floor from the house it is built against. Fine cracks at the junction, in the plaster, or in a new concrete lintel are common while materials lose moisture and the new weight beds in. Surveyors often call that early movement settlement of the new work. That is not the same sentence as subsidence of the Victorian or 1930s house next to it. The Association of British Insurers describes subsidence as downward movement of the ground under a building. Age alone does not prove that, and age alone does not disprove it.
Ask what you were told about the date, and whether any building-control papers were on site. The survey does not replace those papers. If the cracks are wide, stepping through brick, or open at the junction so that weather gets in, say so without jumping to the old house. Do not ask the seller to underpin the main walls because a new extension has shrinkage cracks. If you need someone to say the new footing is adequate, that is an engineer with the design, not a reinterpretation of hairline plaster cracks. The inspection still saw the extension only once.
What should you ask your surveyor?
- Which cracks are in the new extension, and which, if any, are in the original house?
- What age were you given, and did you see anything that supports it?
- Are you describing shrinkage and bedding-in, or are you diagnosing the ground?
What are the typical next steps?
- Keep the extension and the original house as separate paragraphs in anything you send on.
- Ask the solicitor for the approval papers. Do not treat their absence as a crack diagnosis.
- Do not commission underpinning of the main house from cracks that are only in two-year-old plaster.
Costs vary with the building and with who you instruct. Where this page does not cite a published figure, ask the surveyor, a relevant specialist, or your solicitor for a written quote. Do not treat a forum average as the price.
Also asked about this
Do cracks at the junction mean the extension is falling off?
Not by themselves. Junctions crack as the new work dries. Ask how wide they are and whether they are letting water in. The survey has not monitored them.
Does a two-year age prove the movement is harmless?
No. It changes the likely reading. It is not a certificate. Wide cracks or a leaning new wall still need a proper description.