What a double-storey rear extension involves, from brief to handover

A double-storey rear extension adds living space downstairs and bedrooms upstairs in one project. Here is what happens at each stage, and where the important decisions are made.

The short answer: a double-storey rear extension moves through seven stages. The brief, a feasibility check, design, planning and building regulations, structural design, the build itself, and handover. The decisions that most affect cost and quality are made in the first three stages, long before anyone arrives on site.

This guide walks through each stage in the order it happens on a typical project. If you want to see real examples, our Our Work page includes several completed double-storey and wraparound extensions, photographed from the groundworks through to the finished rooms.

Stage 1: What should the brief include?

A good brief describes how you want to live, not just how many square metres you want. Who uses the kitchen and when? Do you need a utility room, a downstairs WC, a home office? Upstairs, is the priority an extra bedroom, an en-suite for the main bedroom, or a larger family bathroom? It helps to collect photos of spaces you like and to be honest about your budget early.

Stage 2: Why start with a feasibility check?

Before any drawings, someone needs to look at the house and answer three questions. Is it structurally possible? Is it likely to get planning permission? What is a realistic budget? A feasibility visit covers the structure of the existing rear wall, the foundations and ground, the position of drains, boundaries and neighbouring windows, and access for the build.

At Everstone, that visit is carried out with a Chartered Civil Engineer's eye, so the structural answer is part of the conversation from the start.

Stage 3: Design

A measured survey of the house is followed by design drawings. For a double-storey extension, the key design questions are the depth of the extension, the roof form and how it meets the existing roof, the position of upstairs windows in relation to neighbours, and how the new rooms connect to the old ones. The ground floor layout usually involves opening up the rear wall, which is where the steel design comes in.

Stage 4: Does a double-storey extension need planning permission?

Most double-storey extensions need planning permission. Some two-storey rear extensions can be permitted development if they are no more than 3 metres deep and at least 7 metres from the rear boundary, among other conditions, but many projects fall outside these limits. Councils aim to decide householder applications within eight weeks. The planning officer will look closely at the effect on neighbours, particularly overshadowing and overlooking.

Separately, the project needs building regulations approval. That covers structure, fire safety, insulation, ventilation and drainage. If you share a wall or are digging close to a neighbour's foundations, party wall notices must be served, normally at least one or two months before the relevant work starts depending on the type of notice.

Stage 5: Why is the structural design so important?

A double-storey extension puts new loads into the existing house. The rear wall is usually removed on the ground floor to create an open-plan space, and the first-floor wall above it, plus part of the roof, then has to be carried on steel beams. Those beams sit on supports that take the load down to foundations, which themselves need to suit the ground.

The new roof also has to be joined to the old one without leaks, cold spots or awkward shapes. Getting these details right is what separates an extension that looks as though it was always there from one that looks added on.

Stage 6: What happens during the build?

1. Site set-up: protecting the house, garden and neighbours, scaffolding and welfare facilities.
2. Groundworks: excavating and pouring foundations, and altering or connecting drainage.
3. Superstructure: walls up to first-floor level, first-floor joists, then walls up to the roof.
4. Roof: structure, covering and the junction with the existing roof, making the extension weathertight.
5. Openings and steel: installing the beams and forming the openings into the existing house, carefully propped.
6. Windows and doors, then first fix electrics, plumbing and heating.
7. Plastering, second fix, kitchen and bathroom installation, flooring and decoration.

Most families stay at home for a double-storey extension. The early stages happen outside, and the break-through into the house is planned so the most disruptive period is kept short. Building control inspects at key points, such as foundations, drainage and structure.

Stage 7: Handover

Before handover we walk through every room with you and agree a snag list. Once any snags are resolved and building control has completed its final inspection, you receive the completion certificate and the paperwork for the work, which you will need when you sell. We will also explain how to look after new finishes, glazing and any heating systems.

What does it cost?

Because double-storey extensions vary so much in size, structure and specification, we do not publish a single figure. At the feasibility stage we give you a realistic range, then an itemised quotation once the design is settled. Remember to budget separately for fees, the kitchen and finishes, and a sensible contingency for the unknowns that only appear once the ground is opened.

How long does it all take?

The design and approvals stages often take as long as the build itself, particularly where planning permission is needed. The build duration depends on the size, the structure and the specification, so we agree a written programme with you before we start.