When Is the Best Time to Build an Extension?

Mar 31

4 min read

Updated: Oct 6

White painted timber conservatory-style extension with a hipped glazed roof on a brick plinth

The best time to build an extension is usually to start on site in spring, which means designing and securing permissions over the preceding autumn and winter. A spring start gives groundwork and structure the kinder months, keeps glazing installation in settled weather, and leaves the finishes for autumn, so the room is ready before the next winter.

For a glass extension or any architectural renovation, the planning and design stages decide the timeline far more than the weather does, as our glass room extension guide explains.

Why timing matters when building an extension

Timing affects planning timelines, contractor availability, build efficiency and how much disruption you live with. The weather matters most for the groundwork and the first weeks of structure; after that, the programme is driven by lead times.

A month-by-month timeline, working back from completion

The easiest way to plan is to fix the date you want to use the room and work backwards. For a spring start and an autumn finish, a typical sequence looks like this:

September to October: survey, brief and concept design. Design work commonly takes four to eight weeks, longer on a listed home.
November to December: planning application or permitted development check, and structural calculations.
January to February: planning decision (usually within about eight weeks of a valid application), building regulations submission and technical drawings.
February to March: tender or confirm the contractor, and order bespoke glazing and joinery once the design is fixed.
April to July: groundwork, foundations, structure and weathertight shell.
July to September: glazing installation, services, internal finishes and making good.

On site, a single-storey extension typically takes somewhere between two and four months, depending on its size, the groundwork and how much of the house is opened up. Ask for a programme before you sign, and build in a margin.

Spring: the most popular time to start building

Spring is widely considered one of the best times to begin. Milder weather and longer daylight help the groundwork and structure, and a spring start leaves time to finish before winter. The catch is demand: builders and designers are busiest for spring starts, so the design, planning application and contractor booking all need to happen over the winter.

Summer: ideal for fast progress and outdoor work

Summer suits large glazing installations, structural openings and landscaping, with minimal weather delays and faster progress on site. For glass extensions, it also lets you judge light and shading as the room takes shape. The drawbacks are that contractors are fully booked and a summer start pushes the finishes towards winter.

Autumn: a strategic choice

Autumn is often overlooked. Contractors may have more availability, though any saving depends on the market rather than the season. Starting in autumn lets the early stages happen before winter, with internal work continuing whatever the weather. The key is to get the weather-sensitive stages, especially groundwork and concrete, done early.

Winter: best for planning and indoor phases

Winter is the preparation phase: design development, planning applications, structural calculations and ordering long-lead items. Building in winter is possible, but frost is the limit. Concrete and mortar should not be laid when frost is expected unless they are protected, because freezing before they cure weakens them, so short days and cold snaps can stop groundwork and brickwork for days at a time. Internal work carries on regardless.

Glazed timber garden building under construction on a stone plinth beside bare winter trees

Planning permission, building regulations and lead times

Many single-storey rear extensions in England fall under permitted development, but a planning application takes around eight weeks to decide, and a larger extension under the prior approval scheme gives the council 42 days. Building regulations apply either way and are a separate approval. The limits and exceptions are set out in our guide to planning permission for glass extensions.

Bespoke glazing is made to order and can take several weeks to manufacture, so it needs ordering once the design is fixed. Ask your designer for current lead times at the start, because they move with demand.

Planning around family life

The build itself is only part of the timing. Think about when the kitchen or the back of the house will be out of use and plan around the dates that matter to you: a summer holiday is a good time to be away during the noisiest weeks, while a build that runs into December can leave you without a finished room over Christmas. School holidays, exams and working from home all change how much disruption a household can take.

What to settle before you book a start date

A start date only holds if the decisions behind it are made. Before you commit, make sure you have:

A fixed design, so glazing, joinery and structural steel can be ordered without later changes.
Planning confirmed, either a decision notice, a prior approval or a Lawful Development Certificate.
Building regulations in hand, with full plans approved or a registered building control approver appointed.
A party wall agreement if you are building on or near a shared boundary, which can take a couple of months on its own.
A written programme from your contractor showing the weather-sensitive stages and the delivery dates they depend on.
Your budget settled, including a contingency, so choices are not reopened mid-build.