Most people asking this question are looking at one sketch and wondering whether it is too much. It is easier to judge with two or three different shapes side by side, holding the same house and the same garden. This page does that on one worked example, then lists what to check before you take a shape further.
The house and the garden
The example is a detached two-storey house, 8.0 m wide and 9.0 m deep, with eaves at 5.2 m and a ridge at 8.0 m running parallel to the street. It sits on a 10 × 30 m plot, 1.0 m from each side boundary, with a 15 m deep rear garden of 150 m². These numbers are illustrative, chosen to make the comparison clear; yours will differ.
Three additions, compared
- A two-storey box across the full width, 8.0 × 4.0 m, with a flat roof at 5.6 m: slightly above the house's eaves.
- A single-storey addition across the full width, 8.0 × 4.0 m, 3.0 m high.
- A narrower two-storey wing, 4.0 m wide and 5.0 m deep, set 1.0 m in from one side wall, with its roof at 5.0 m: below the eaves.
| Measure | 1 · Two-storey, full width | 2 · Single-storey, full width | 3 · Narrow two-storey wing |
|---|---|---|---|
| Added floor area (gross) | 64 m² | 32 m² | 40 m² |
| Height of the addition | 5.6 m, above the 5.2 m eaves | 3.0 m | 5.0 m, below the eaves |
| Width against the 8.0 m house | Full width | Full width | Half (4.0 m), 1.0 m in from the side |
| Rear garden left (of 150 m²) | 110 m² (−27%) | 110 m² (−27%) | 130 m² (−13%) |
| Shallowest garden depth | 11.0 m | 11.0 m | 10.0 m behind the wing, 15.0 m beside it |
| Upstairs rear rooms | Lose their outlook to the extension | Keep their outlook | Half keep it |
| How it reads | A bigger house | A clear addition to the house | A smaller building beside the house |
Areas are gross, to the outside face of the walls. Garden figures are the rear garden of the illustrative plot only. Heights are the top of each addition's roof above the ground floor.
Why the same idea looks so different
Height: above or below the eaves
The eaves line is the strongest horizontal on most houses. An addition that rises above it, like option 1, merges with the main volume and makes the whole house look bigger. One that stays below it, like options 2 and 3, keeps the original outline readable, so the addition reads as an addition.
Width: full width or part of it
A full-width addition extends the house in every view from behind. A narrower wing leaves part of the original rear wall in sight, keeps light and outlook for the rooms beside it and takes less garden. Option 3 uses half the house's width and still adds 40 m², more than the single-storey option.
Setting in and setting back
Pulling an addition in from the side walls, or back from the front, creates a step that separates old from new. Option 3's 1.0 m step also moves its side wall 1.0 m further from the neighbour, which reduces how much it looms over the boundary.
Depth and the garden
Every metre of depth comes straight out of the garden and moves a wall closer to whoever lives behind. Options 1 and 2 cost the same garden (40 m²) for very different floor areas; option 3 costs half as much ground and keeps a 15 m deep strip of garden beside it.
Splitting into parts
When one addition would be too big, two smaller ones, such as a single-storey kitchen across the back and a modest upstairs wing over part of it, can carry the same area while each stays secondary. The trade-off is more corners, more roof edges and more to build.
What local guidance usually looks for
Rules differ by country and by authority, so treat this as a pattern, not a rule. Many local planning authorities publish design guidance for householders, and a recurring idea is that an extension should be subservient, or secondary, to the original house. As one example from England, Coventry City Council's Householder Design Guide (2023) lists ways to achieve that, including a lower ridge height, setbacks, and keeping an extension's width to no more than half that of the existing house. Your authority may say something different, and whether you need permission at all is a separate question: check your own authority's current guidance, or ask an architect or planning consultant.
What to check before you go further
- Inside, not only outside. Does the new space connect sensibly to the existing rooms, or does it turn a room into a corridor? A floor plan answers that; the shape cannot.
- Light into the existing house. A deep addition darkens the rooms it is built against. Note which existing windows it covers.
- The neighbours' side. Draw the elevation they will see, with the boundary in it.
- The roof. Every flat top needs a decision: a roof, a terrace with a balustrade, or something else.
- The rules where you live. Heights, depths and distances to boundaries are local questions with local answers.
For the wider job of planning a renovation, including drawing the existing house first, see Plan a renovation and the worked renovation options example. For comparing whole building shapes on an open site, see How to compare building shapes.