How Long Does a Two Storey Extension Take?

Sixteen to twenty six weeks on site, and seven to nine months from the first survey to the completion certificate. The building work is roughly double a single storey extension. The paperwork in front of it is more than double, and that is where the extra months actually sit.

One line in the permitted development rules explains most of it. Almost every two storey scheme needs a full planning application, and that is eight statutory weeks a single storey extension can often skip entirely.

The front end is the part that grows

A two storey extension is the commonest domestic project that begins with somebody at the council making a decision about it.

Under Class A, a rear extension of more than one storey stays within permitted development only if it projects no more than 3 metres from the original rear wall and comes no nearer than 7 metres to the boundary opposite that wall. The roof pitch has to match the original as closely as is practical, and upper floor windows in a side elevation have to be obscure glazed and fixed shut below 1.7 metres. On a plot with a garden of ordinary depth the 7 metre rule is the one that fails first. On designated land a two storey extension sits outside Class A altogether.

So the realistic front end is measured drawings, a structural design, a householder application and a Building Regulations submission, all run alongside each other. Three months is fair for that, against something closer to two for a single storey. The eight week determination period starts at validation rather than at submission, and a resubmission after a refusal starts a fresh eight weeks rather than resuming the old ones.

Party wall notices are the other addition, and on half of a semi they are close to unavoidable. Floor beams bearing into the shared wall, and foundations dug within 3 metres of it, are both notifiable. That is a two month period, shortened only by the adjoining owner consenting in writing. How the Act works in practice is worth reading before a notice is drafted, because serving late is what creates the delay rather than the neighbour.

On site, and where the range lives

Two things set the site programme, and neither is the floor area.

The first is underground. Foundations for two storeys carry roughly twice the load, so the trench is wider before anything is said about depth, and on shrinkable clay near a mature tree the depth is set by what the subsoil is doing rather than by the drawing. More concrete, more spoil, more days.

The second is the roof. A single storey extension has a roof that lands against a wall. A two storey extension has one that has to be married into the existing roof, which means stripping tiles back, forming a valley or an abutment, and rebuilding a section of something that was previously watertight. That is skilled work at height, it is exposed to weather from a scaffold rather than from the ground, and it is the stage where a bad fortnight of wind does most damage to a programme.

Between those two, the middle is fast and looks it. Blockwork to first floor, beams in, floor deck down, blockwork to eaves, roof on. A fortnight where a great deal appears to happen, because it does.

What holds it up, in order of how often

• Steel. A two storey extension has more of it and it is more interdependent: floor beams, opening beams, and padstones sized to suit. It is fabricated to a schedule, and that schedule cannot be issued until the structural design is signed off. Ordered off approved calculations in the first week it is invisible. Ordered when the walls are up it stops the job dead.
• Inspection holds. Building Control has to see the trench before concrete, the drains before backfill, the beam bearings, the damp proof course and the insulation. Each is a day rather than a week, but work that has not been seen cannot be covered.
• Scaffold adaptations. The scaffold is lifted as the shell rises and altered again for the roof. Each visit needs booking, and a scaffolder who is elsewhere costs a day at a point where nothing else can proceed.
• Making good inside. Every room the new floor touches gets plastered, rewired at the edges and decorated. That work is inside the programme whether or not anyone put it in the budget.

The weeks that happen inside the house

This is the real difference from a single storey job, and it is a difference in character rather than in duration. A single storey extension is built outside the wall it will eventually join, so the house stays intact behind its old back wall for nearly all of it.

A two storey extension opens the first floor. For a period there is a hole in the back wall on two levels, the bedrooms above it are unusable, and scaffold stands against the whole rear elevation with sheeting on it. That period is a fortnight or so rather than a season, it is planned with you in advance, and it is the single thing worth asking about before you decide whether to stay. What living through the rest of it is like has a longer answer of its own.

The honest total

Those figures assume the front end runs in parallel. Run sequentially, waiting for the decision before commissioning the structural design and waiting for that before serving notices, the same job takes closer to a year without anybody doing anything wrong.

The comparison worth making is with the alternative rather than with a single storey. If what you need is bedrooms, a loft conversion reaches the same result in roughly half the calendar time and takes no garden. Where a two storey extension earns its months is when you want the ground floor bigger as well, because the foundations, the scaffold and the roof are being paid for once either way. The full sequence from survey to certificate sets out how the stages fit together, and how we sequence a two storey build covers what happens week by week.

Programme figures here are typical for the Colchester area at the review date below. They are ranges with reasons attached, not promises.

Last reviewed August 2026.