Part O: Simplified Method or Dynamic Modelling?

Approved Document O applies to new residential buildings in England. It gives you two ways to show a dwelling will not overheat, and the choice between them is worth making deliberately rather than by default.

Route one: the Simplified Method

The Simplified Method is prescriptive. It sets maximum glazing areas and minimum openable free areas, varying by:

• Orientation: south and west facades are treated more strictly
• Dwelling type: cross-ventilated or not
• Risk location: high or moderate, by postcode

You check your dwelling against the tables. If it is inside every limit, you have complied.

Three things people get wrong:

1. You cannot interpolate or average the table values. The standards apply as written.
2. Where two facades have equal glazing, you use the orientation that gives the more stringent standard.
3. Glazing g-values are centre-pane values and exclude the frame.

For a compact house with ordinary windows, this route is quick and it is enough. For many plots on many sites, it is all the evidence anyone needs.

Route two: dynamic thermal modelling

The second route is a full dynamic thermal simulation following CIBSE TM59, assessed against both tests, with a report and the Appendix B compliance checklist for the Building Control Body.

You are no longer proving the dwelling sits inside a set of prescriptive limits. You are demonstrating how it actually performs, hour by hour, across a design summer year.

When the simple route stops being the cheap route

The Simplified Method is fast, but it buys that speed with design freedom. It becomes the expensive option when the limits start dictating the architecture rather than confirming it.

Watch for:

• Single-aspect flats. No cross-ventilation means a much higher free-area requirement, and one elevation to get it from.
• Corner flats. Windows on two elevations do not automatically make a dwelling cross-ventilated.
• West-facing glazing. The limits here are tight, and bi-folds or floor-to-ceiling glass will often breach them.
• High-risk locations. Urban postcodes carry stricter standards.
• Restricted openings. Noise, security and fall protection all reduce the area that counts.

In each case you have the same choice: redesign to fit the table, or model the building as drawn.

The expensive mistake

The costly pattern is this. Someone runs the Simplified Method late, it fails by a small margin, and the development goes out for a TM59 assessment at short notice, paying for both and finding out after the drawings are fixed.

The cheap pattern is the reverse. Run the Simplified Method on every house type at concept stage. It takes minutes. Types that pass comfortably are done. Types that fail, or that only pass by shrinking windows you wanted, go to dynamic modelling while the design can still absorb the answer.

One thing worth being clear about

Neither route promises the home will never be too warm. Both reduce risk and demonstrate compliance. The Simplified Method in particular is a set of proxies. Passing it means your dwelling resembles dwellings that tend not to overheat, not that yours has been tested.

If you care about the outcome rather than the certificate, dynamic modelling tells you considerably more.

AutoTHERMAL runs both routes from the same model, so moving between them costs you nothing. [Join the waitlist](/waitlist).