The quick answer
A double-storey extension adds a room on the ground floor and another directly above it, built as one structure and roofed to match the main house. Compared with building out on one level, you gain roughly twice the floor area on the same footprint, but the planning test is stricter, the foundations carry more load and the roof has to be married into the existing ridge or eaves. Most of the decisions that shape cost and timing are made before anyone digs a trench.
Why people go up as well as out
Garden space is usually the limit. Going up lets a family add a bedroom, a bathroom or a larger landing without losing more lawn than a single-storey addition would take. On semi-detached houses a side addition over two floors is a common way to add a fourth bedroom while also widening the kitchen underneath.
It also tends to be more efficient per square metre. The groundwork, foundations and roof are needed whether you build one floor or two, so the second floor shares those costs rather than repeating them.
Planning is the first hurdle
Two-storey additions are much less likely to fall under permitted development than single-storey ones. Rear two-storey work under those rights is limited in depth and has to keep a set distance from the rear boundary, and side two-storey additions nearly always need a full householder application. Councils look closely at overlooking, loss of light to neighbouring windows and how the new roof sits against the old one.
In conservation areas the design is judged more carefully again, with attention to brick type, window proportions and roof pitch. Getting drawings prepared with those points in mind usually saves a round of amendments.
Structure: foundations, steels and walls
Because the new walls carry an upper floor and a roof, the footings are sized for that load and for the ground conditions found on the day. Clay soils, nearby trees and old drains all affect how deep the trench goes, and Building Control inspects the excavation before concrete is poured.
Where the extension opens into the existing house on both floors, steel beams replace the old external wall. An engineer designs those beams and their padstones, and the calculations form part of the Building Regulations approval. Upstairs, the new floor joists are hung from or supported alongside the existing structure so the two floors line up without a step.
The roof tie-in
The roof is where a two-storey extension succeeds or looks bolted on. Ideally the new ridge sits slightly below the main ridge and the eaves line continues round, so the addition reads as subordinate to the original house. Tiles, lead valleys and flashings have to be detailed so rain runs off cleanly where the two roofs meet.
The order of work on site
A typical sequence runs: set-out and excavation, footings and inspection, brick-and-block walls to first-floor level, upper floor joists, walls to the eaves, roof structure and covering, then windows so the shell is weathertight. Only once it is dry do we break through into the house, fit the steels and connect the rooms. First-fix services, plastering and second fix follow.
Keeping the breakthrough late means you live in a sealed house for most of the build, with the mess and dust confined to the new structure.
Matching the brickwork
A two-storey wall is a large, very visible area of new brick, so the match matters more than on a low rear addition. We look at the brick colour, texture and size, the mortar colour and the joint profile, and lay test panels where the existing brick is hard to source.
How we can help
D & A Brickwork & Building is a family-run firm based in Bidwell, Dunstable, with over 26 years of building experience, NVQ Level 2 bricklaying, FMB membership and full insurance. We work from your architect's plans or can recommend one, and we project-manage the build from foundations to finish. The cost calculator on this site gives a rough starting range, and a free written quote follows a site visit.
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Questions people ask
Does a double-storey extension need planning permission?
Usually. Side two-storey additions almost always need an application, and rear ones only qualify as permitted development within tight depth and boundary limits. Check with your council or a designer before committing.
Do I need an engineer?
Yes. The steels that open the new rooms into the house, and often the foundations, need calculations for Building Regulations approval.
Can we stay in the house during the build?
In most cases, yes. The shell is built and made weathertight before the existing wall is opened, which keeps disruption to the later stages.
Will the new brick match the old?
We match colour, texture, size and mortar as closely as possible and use test panels where the original brick is hard to find.