A well-drawn extension starts long before you open your CAD package. It starts in the garden with a tape measure, a laser distance meter and a phone full of photographs. Getting the survey right is the single biggest favour you can do yourself, because every later decision — the wall build-up, the roof pitch, the position of the drainage run — depends on knowing exactly where your plot really begins and ends.

Start With the Boundary, Not the Building

Download or dig out the title plan and treat it as a guide, not gospel. Title plans are indicative; fences, hedges and the physical boundary may sit half a metre either side of the dashed red line. Only a measured survey settles the question.

Work around the perimeter first, recording distances from each boundary to the existing house at several points, then triangulate with diagonal checks. If your longest measurement and your shortest disagree by more than about 20mm over five metres, measure again. Record ground levels relative to the damp-proof course as well as to a fixed datum — a timber post, a manhole cover, a stable corner of the house. A fall of 150mm across a garden looks like nothing on site and looks enormous in a cross-section.

Party Walls and the Neighbours' Rights

For terraced and semi-detached homes, the shared wall is usually the biggest unknown. Victorian solid brickwork might be 215mm, 225mm or 325mm thick, and the face you measure inside may not be the face your neighbour sees. Measure from the internal face to the boundary centreline and mark it clearly on your survey drawing, because that centreline governs how far your extension can project.

The Party Wall etc. Act 1996 applies if you excavate within 3 metres of a neighbouring structure and go deeper than their foundations, or within 6 metres in certain cases. It also applies if you build into or cut into a party wall, or raise it. Design the setting-out with these distances in mind and note them on the drawing. Your CAD file is often the first place a party wall surveyor will look.

Reading the Permitted Development Rules Into Your Drawing

Most small rear extensions fall under permitted development, but the limits are strict and they vary with house type. For a single-storey rear extension, the usual figures are:

• Depth: up to 3 metres for a terraced or semi-detached house, up to 4 metres for a detached house (longer under the prior approval neighbour consultation scheme — up to 4 metres and 8 metres respectively).
• Height: a maximum of 4 metres overall, with eaves no higher than 3 metres.
• Boundary rule: if the extension is within 2 metres of the boundary, the total height must not exceed 3 metres.
• Materials: they must be of a similar appearance to the existing house.

Two-storey extensions are tighter still — typically 3 metres deep, with a roof pitch matching the existing house and eaves that align. Some properties lose these rights entirely: flats, maisonettes, listed buildings, and houses covered by an Article 4 direction, which is common in conservation areas. Draw the permitted development envelope as a faint dashed outline in the same file so you can see at a glance whether your proposal breaches it — it is far easier to adjust a line than to rework a planning application.

Setting Up a Clean CAD File

Draw everything full size in millimetres in model space and use layout tabs for sheet setup. Create separate layers for existing fabric, proposed fabric, demolished elements, boundary, dimensions and annotation, and keep the line weights purposeful: heavy for cut walls, medium for projections, light for hatching and setting-out.

Build your walls as real assemblies rather than single lines. A typical modern cavity wall is around 300mm: 102.5mm brick outer leaf, 100mm cavity with insulation, 100mm blockwork inner leaf, plus plasterboard. Getting this right at the outset means your internal dimensions, door openings and structural spans all hold together later.

Sections, Elevations and Drainage

A plan alone rarely convinces anyone. Draw at least one long section through the extension showing the foundation depth, the damp-proof course, the floor build-up, the eaves and the roof pitch. For a tiled roof, 35 to 45 degrees is the normal band, and the pitch must clear the head of any first-floor window you are abutting.

Check the drainage next. New gullies, rainwater pipes and any diverted runs need falls of roughly 1 in 60 to 1 in 80, and you should show the position of existing manholes, rodding eyes and any soakaway. If a run crosses the extension footprint, that is a design change, not a note on a drawing.

The Package You Hand Over

Finish with a set that a planner, a builder and a building control officer can all read without a phone call: a location plan, a block plan at 1:1250 or 1:500, existing and proposed site plans at 1:200, general arrangement plans at 1:50, four elevations, two sections, and a short schedule of materials and finishes.

Keep the existing and proposed states visibly distinct, dimension the key setting-out points from the boundary, and label the drawing with the property address, scale, date and revision. Do that, and your drawings will do the persuading for you.