Combining More Than 4 Extensions

Up to four extensions can be defined in RdSAP. 

This makes it possible to model a wider range of less typical building layouts, such as mezzanine levels or mixed roof constructions, in addition to standard extensions. Vertical extensions can also be recorded explicitly – for example, those built above a garage or on top of an existing storey rather than at ground level.

However, on some occasions you’ll assess a property more than 4 extensions.

As the software only allows up to 4 extensions to be entered, you’ll need to look at combining the most similar extensions together.

Convention 2.12 states what you should do when you have more than 4 extensions:

Add together floor areas and exposed perimeters of extensions (or add extension to main dwelling) to reduce to four extensions. Combine parts having the most similar age bands and thermal/construction characteristics (refer to Section 6 of RdSAP 10 for U-values of relevant constructions). Use alternative wall where appropriate. (Convention 2.12)

When a dwelling contains multiple areas with differing construction elements, it is necessary to group together those with the most similar characteristics and age bands so that no more than four extensions are used.

Section 6 of the RdSAP 10 document provides U-values for relevant constructions, which should be used to determine the most appropriate combinations. Where suitable, the alternative wall option can also be applied.

Note: Extensions combined in this way do not need to be physically adjacent, provided the key factors – floor area, ceiling height, and total heat loss perimeter – are recorded accurately.

As noted above in Convention 2.12, it is also possible to add the extra extension(s) to the main dwelling if necessary. 

Let’s take a look at the example below to find out which we should combine in this instance.

Example: A 1960's Bungalow that has been extended on 3 occasions.

In this example we have a bungalow built in the 1960’s with MAIN, EXT1 and EXT2 forming part of the original property.

The property has later been extended to the side and a porch has been added that is not thermally separated. The property is detached, and of cavity wall construction.

Here’s an overview of the property relevant to combining the extensions based on similar characteristics:

MAIN: Suspended Floor, Pitched Roof, 100mm Loft Insulation, 1960s

EXT1: Solid Floor, Pitched Roof, 100mm Loft Insulation , 1960s

EXT2: Sold Floor, Flat Roof, 1960s

EXT3: Solid Floor, Pitched Roof, 250mm Loft Insulation, 1990’s

EXT 4: Solid Floor, Pitched Rood, 400mm Loft Insulation, 2025

EXT 5: Solid Floor, Flat Roof, Estimated as 1970’s

We have 5 extensions at this property, but which should we combine together?

Given that the main house and EXT1 both have pitched roofs and the same levels of insulation and build date - we are likely to choose to keep these separated, with the difference being the floor type.

The later extensions to the side were built in the 1990s and 2025 - so will be to different standards of building regulations than the original property (and different U-Values) - so it’s likely that they should remain separated.

However, we have 2 extensions, with similar characteristics. EXT2 and EXT5 both have solid floors and flat roofs, but are built during different age bands.

The age bands (and U-Values) are relatively close to each other in this example, so it makes sense that these extensions should be combined.

When entering the data into the software with regards to the calculations we simply add these combined extensions together for area and heat loss perimeter (and party wall if applicable).

For the height, we can take the average weighted ceiling height of both extensions.

We will also need to update our floorplan showing that EXT2 now includes the porch. See below:

Now we have 4 extensions instead of the original 5 on our floorplan.

This process is similar if you had 6, 7 or more extensions. Combine the most relevant in order to reduce the number of extensions to 4. 

Note: Ensure you add a note to your assessment to clearly state which extensions have been combined, so anyone looking at it later (including yourself) can understand what has been combined.