A new extension requires attention in its first year in ways that are easy to miss if you're not looking. The building is settling, materials are drying out and adjusting to temperature cycling, and minor issues that appear in year one are almost always cheaper and easier to address when caught early than when left until they develop into something more significant. This guide sets out what to watch for and what maintenance routines genuinely protect the investment you've made.
The First Year: What's Normal and What Isn't
New buildings settle. The foundations compress fractionally under load, the masonry adjusts to moisture cycles, and timber dries and moves. In a well-built extension on sound foundations, this produces nothing more than hairline shrinkage cracks in plasterwork and possibly slight movement at door frames. These are cosmetic and expected.
What's not normal in year one: cracks that are widening rather than stable, diagonal cracks spreading from window corners outward into brickwork, sticking doors that weren't sticking before, or uneven floors that develop a new slope or bounce. If any of these appear, have them assessed rather than assumed to be normal settlement. Most will have an innocent explanation, but some won't.
Monitor any cracks you notice by marking their ends with a pencil line and a date. If the crack extends beyond the mark over two or three months, it's active movement and needs professional assessment. If it stays within the marks, it's settled and can be filled and forgotten.
External Maintenance in Year One
Check gutters and downpipes after the first significant rainfall. New gutters can be installed correctly and still develop leaks at joints, particularly after they've gone through thermal expansion cycles. A leaking gutter joint that deposits water against the wall can cause damp within months. Check from ground level during heavy rain, and walk round after dry weather when water stains on render or brickwork are visible.
Check roof junctions and flashings. Lead flashings to the junction between the new roof and the existing house wall are a common source of water ingress. They can lift or crack in their first winter as the lead adjusts to temperature cycling. A visual inspection from outside in the autumn, and again after any particularly cold or windy period, takes ten minutes and catches problems early.
Silicone seals around windows and doors. New silicone can shrink slightly and pull away from the masonry in its first year, particularly if applied in warm weather and then exposed to cold. Run a finger along the bead around all windows and external door frames. Any point where the silicone has separated from the reveal or frame should be cut out and resealed.
External render or cladding. New render needs time to cure before painting and can develop hairline cracks in the first year, particularly cement render on a new extension that's adjacent to old masonry (differential movement rates). Small cracks should be filled and the area painted. Cracks that are opening or that run continuously across a large area need assessment.
Services Checks
Boiler service. Any new boiler should be serviced annually by a Gas Safe registered engineer. The manufacturer's warranty typically requires documented annual servicing to remain valid. The first service, usually 12 months after installation, confirms that the system is working correctly and addresses any minor commissioning issues.
Heat pump service. Air source heat pumps require annual maintenance: cleaning the filter and heat exchanger, checking refrigerant levels, and inspecting electrical connections. Most MCS-accredited installers offer service contracts. The Boiler Upgrade Scheme grant comes with requirements to maintain the system appropriately.
MVHR filters. Mechanical ventilation with heat recovery systems have filters that need replacement every 3-12 months depending on the system and location. A blocked filter reduces the system's efficiency significantly. Most systems have an indicator light or app notification when filters need changing.
Underfloor heating zones. Check all UFH zones are responding correctly by the end of the first heating season. Zones that don't heat up may have air in the circuits (addressable by bleeding via the manifold), an actuator that's stuck, or a wiring issue at the thermostat. These are all minor issues when addressed promptly; left for a year they can become more difficult to diagnose.
Long-Term Maintenance Schedule
Beyond the first year, a new extension needs the same routine maintenance as any other part of the house, plus some specific items:
Every year: Clear gutters and downpipes in autumn (leaves from adjacent trees accumulate and cause overflow). Boiler or heat pump service. Check external silicone seals. Inspect flat roof surfaces if applicable (look for ponding water, membrane splits or blisters, debris in the drain outlets).
Every 5 years: Repaint timber windows and exterior timber if applicable. Have the electrical installation checked (EICR recommended every 10 years, every 5 for rental). Check the condition of external render or cladding.
As required: Repoint brickwork where mortar has deteriorated (inspect every few years; repointing becomes necessary when mortar is soft, crumbling, or recessed more than 10mm). Check the condition of the roof covering (slates, tiles, or flat roof membrane) every few years; moss and lichen accumulation is normal on pitched roofs but can be treated with biocide to prevent moisture retention.
Keep the handover documents somewhere accessible. Operating manuals for boilers, heat pumps, MVHR systems, smart home controls, and any specialist equipment are usually forgotten about until something goes wrong. Filing them physically in a folder, or scanning them as PDFs, and knowing where they are means the service engineer can see the full system history rather than starting from scratch.
Raising Defects During the Liability Period
Most building contracts include a defects liability period of 6 to 12 months from practical completion during which the contractor is obliged to return and repair any defects in their work. Use this period proactively, not reactively.
Keep a list of anything that doesn't seem right throughout the year: a door that sticks in winter but opens in summer (normal seasonal timber movement in its first year; if it persists, a defect), a toilet that runs slightly overnight, a section of skirting board that's separated from the wall. Compile these at 9-10 months and raise them in a single written notification to the contractor before the liability period expires.
Raising everything at once is more efficient for the contractor and produces a single agreed rectification visit rather than multiple call-outs. The key is to notify within the liability period: once the period has expired, you've lost the right to have the contractor return at their cost, and the work becomes a repair at your expense.