Last month, my wife Ewa and I found ourselves in a bit of a quandary while preparing plans for our new extension. I was keen to try my hand at preparing the detailed drawings for our Building Regulations’ application, but with professional CAD packages costing well into four figures, the sums simply didn’t add up.

Rear elevation of Ian Rock's proposed extension

Factor in the cost of your own time, and it makes a lot more sense to appoint a professional to do the job for you. But in this instance, I was determined to learn a new skill and prepare my own drawings.

After more research, my hopes were raised when I came across a possible solution — Arcon’s 3D Architect Designer software looked like it might fit the bill, with a range of packages priced from £150 to £550. After speaking to Tim Bates at Elecosoft (the company behind the software), I took the plunge and opted for Arcon Evo, a well-reviewed CAD package specifically geared to ‘building design and floorplans’.

After five days of self-imposed ‘study exile’ I emerged pale and blinking into the daylight, proudly clutching a set of professional-looking drawings. My efforts were greatly assisted by the facility to import existing scale drawings and trace over them using guidelines, thereby ensuring continuity with the planning drawings.

Creating Detailed Drawings

To my great satisfaction, it was now possible to print out multiple copies whenever required. Although the same result could have been accomplished in a fraction of the time by drawing plans by hand, the real bonus comes if you need to make any amendments, produce working drawings for the builders, or produce more detailed drawings of specific areas, such as foundations or floor joists, using an earlier copy as a template.

To avoid cramming all the details on one A1- or A2-sized sheet, I submitted the following additional information on separate documents:

Thermal insulation compliance

This lists the U values you aim to achieve for each thermal element of the building — the walls, roofs, windows, doors and floors. The performance obviously depends on the choice of construction materials and how easily heat passes through them (the ‘lambda-value’ or ‘K-value’) as well as the type and thickness of the insulation. Fortunately, there are plenty of online calculators available, such as the one on Kingspan Insulation’s website.

Span tables

The span and spacing of floor joists, roof rafters and flat roof joists will normally be calculated by your structural engineer, although there are span tables online that you can view for reference (try Alternatively, if you provide the manufacturers of I-joists and beam and block floors with your drawings, they should be able to provide the necessary calculations free of charge.

Window-to-floor ratio

The combined area of all new glazing in an extension (e.g. windows, doors and rooflights) should total no more than 25% of the internal floor area (after deducting any existing openings covered over by the extension from the total area of new glazing). The proposed total area of new glazing on our extension, at 35m2, far exceeds the 28m2 allowed to us under the ‘25% rule’ (based on 112m2 gross internal floor area).

Happily, factoring in the total area of existing windows and doors engulfed by the extension buys us an additional 7m2 — just enough to comply! Where your design exceeds this, you will need to provide some form of justification, usually achievable by beefing up thermal efficiency elsewhere in an extension.

Time-Saving Measures

The job of the drawings is to tell building control in diagram and text form how each specific part of the building is going to comply with the Buildings Regulations. But no matter how much of a CAD genius you are, there’s no point trying to illustrate complex design details that already exist in diagrammatic form. Fortunately, Arcon programs can import ‘ready-made’ detailing.

There is an excellent website for this purpose,, where, for a small fee, you can download a wide selection of diagrams and incorporate them in your plans. This saves you having to scour through the Approved Documents (the Department for Communities and Local Government’s hefty guidance on ways to meet Buildings Regs) to try and locate relevant diagrams. You also get access to a lot of useful Building Regs compliance text which can be copied and pasted to populate your drawings (and edited if required).

In the end, our completed application comprised a set of 16 A3-sized drawings (mostly 1:100 scale). In addition to those showing the main elevations, floorplans, sections and site layout, it was a simple job to print off additional sheets with details of drainage, ventilation and floor structures — thanks largely to the availability of ready-made diagrams and pre-formulated chunks of text.

Submitting Plans to Building Control

In my local authority, the fee payable for a Full Plans application depends on the projected floor size. As it was, we ended up with a total bill of £880, comprising an initial £320 charge payable up front, with the balance paid as an ‘inspection charge’ around the time of the first site visit. The alternative Building Notice route is now 20% more expensive, presumably in a bid to put people off using the ‘short cut’ method.

There was just one key document left outstanding, probably the most important of all — the structural engineer’s calculations. However, before our drawings could be completed there were a couple of structural issues I first needed to get some advice on. My next task was to find a tame structural engineer…


The fruits of Ian’s labour — one of 16 drawings that he was able to create in his new CAD software. This one shows the proposed elevations of the extension


The proposed drainage plans for the ground floor of his extension

  • Pete James

    Perfect, just what I am looking to do myself on a smaller scale.
    Good luck and thank you for the write up.

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