# Estimated dwelling stock methodology

Latest release
Reference period
June Quarter 2022
Released
31/10/2022
Next release Unknown
First release

## Estimated dwelling stock

Estimated dwelling stock are based on adjusted counts from the 2021 Census of Population and Housing (the Census), updated with quarterly estimates of dwelling additions and removals. Further information on each component can be found below.

This release includes estimates of dwelling stock, additions and removals for the June 2016 quarter to the June 2022 quarter.

Development funding for these statistics was provided through the National Housing and Homelessness Agreement, contributing to indicator (a) under the Data Improvement Plan. Development funding ended in 2021-22.

### Scope

Estimated dwelling stock and its components of change (additions and removals) includes all dwellings as defined by the Functional Classification of Buildings (FCB). This defines a dwelling as a permanent and fixed structure intended for long-term residential use, including cooking and bathing facilities. This differs from the definition of a dwelling used by the Census (a structure which is intended to have people live in it, and which is habitable on Census night), which can include non-permanent structures such as caravans, houseboats and tents.

Dwelling stock estimates exclude Non-Private dwellings (NPDs) as defined in the Census (establishments which provide communal and short-term accommodation e.g. hospitals, hotels), which are not classified as dwellings in the FCB. Dwelling stock estimates include both Private and Public/Government owned dwellings.

The components of estimated dwelling stock change (additions and removals) classify dwellings according to the original intended function of the building at the time of construction. The Census classifies dwellings based on the building’s use on Census night. Building use can change over time (i.e. from a short-term holiday apartment to a long-term residential dwelling). Changes in building use that occurred between Census 2016 and 2021 have been added into the components of dwelling stock change.

### Geographic coverage

Dwelling stock and its components of change includes all dwellings in Australia excluding Other Territories, as defined by the Australia Statistical Geography Standard (ASGS) Edition 3. Estimates for Other Territories are not provided due to lack of data availability.

Estimates are prepared at the Statistical Area Level 2, according to the ASGS.

### Dwelling classifications

Dwelling stock estimates are classified by type of building according to the Functional Classification of Buildings (FCB). Dwelling stock estimates are available for the following building types:

• Houses - detached buildings consisting of one dwelling unit, including transportable houses and detached granny flats
• Townhouses - dwellings attached in some structural way to one or more other dwellings, with their own private grounds and no other dwelling above or below
• Apartments - blocks of dwellings that don't have their own private grounds and usually share a common entrance, foyer or stairwell, including attached granny flats
• Total - includes dwellings in non-residential buildings (e.g. caretaker/manager's residence and house/flat attached to a shop)

In the Census, dwellings are classified according to Dwelling Structure (STRD), which largely aligns with the FCB. There are some STRD classes which do not have a corresponding class in the FCB:

• STRD 91 (Caravan) and STRD 92 (Cabin, houseboat) are included in dwelling stock estimates in the houses category, where they have been identified as fixed structures used for long-term residential purposes. These have been identified based on the Census location of dwelling (DLOD) variable (those in retirement villages (DLOD 4) or manufactured home estates (DLOD 3) are in scope of dwelling stock estimates) and information from ABS field officers. Houseboats are excluded from dwelling stock estimates. Caravans and cabins that are used for short-term accommodation (i.e. in holiday parks) or are mobile (i.e. a caravan where someone usually lives but is moved regularly between sites) are not in scope of dwelling stock estimates.
• STRD 93 (Improvised home, tent, sleepers out) is excluded from dwelling stock estimates.
• STRD 94 (House or flat attached to a shop, office, etc.) is included in dwelling stock estimates in the total type of building category only.
• STRD && (Not stated) has been prorated to STRD and DLOD by meshblock.

### Method

The 30 June 2021 Census-based estimated dwelling stock figure forms the base for the quarterly estimates going forward, and has been used as the base for the estimated dwelling stock figures going backwards to the June 2016 quarter.

The 30 June 2021 estimated dwelling stock is calculated by:

• adjusting Census counts for scope differences between Census and construction data (see scope above),
• adjusting for additions and removals from 30 June to Census night (10th August 2021), and
• applying any other necessary adjustments based on coherence with other data sources and to remove implausible changes in dwelling stock.

Dwelling stock estimates for September 2021 quarter onwards are calculated by taking the dwelling stock estimate at the end of the previous quarter and adding the components of change (dwelling additions minus dwelling removals), as follows:
$$D_{t+1}=D_t+A-R$$  where:
$$D_t$$ = the estimated dwelling stock at time point t
$$D_{t+1}$$ = the estimated dwelling stock at time point t+1
$$A$$ = the number of additions to stock occurring between t and t+1
$$R$$ = the number of removals from stock occurring between t and t+1

Dwelling stock estimates for the June 2016 quarter to the March 2021 quarter are calculated by taking the dwelling stock estimate at the end of the subsequent quarter and removing the components of change, as follows:
$$D_t=D_{t+1}-A+R$$

There will be some differences between estimated dwelling stock for the June 2016 quarter and Census 2016 data. These differences may be due to:

• timing differences between 2016 Census (August 9) and 30 June 2016
• improvements in coverage or corrections to dwelling classifications in the 2021 Census
• error in the estimates of dwelling additions or removals (see further information on each component below)
• possible under- or over-coverage in the Census (in each of 2016 and 2021)

Additions to stock are based on dwelling completions from the Building Activity Survey (BACS), which provides quarterly dwelling completions at state level. Dwelling completions include construction of new dwellings and dwellings created as a result of alterations/additions to existing buildings.

BACS is a quarterly sample survey which uses Building Approvals (BAPS), a monthly administrative collection, as the selection frame. BACS collects information (including the completion date) quarterly from builders until the building job reaches completion or is abandoned. All building approvals with over nine dwellings or valued over \$5m are selected in the BACS sample. See Building Activity, Australia methodology for further information.

Small area dwelling completions are compiled from:

1. Directly collected dwelling completions (building approvals selected in the BACS sample), which are allocated to the SA2 from the building approval and allocated to the quarter in which they were completed as reported by the BACS respondent. Approximately 40% of dwellings approved are selected in the BACS sample (8% of houses, 45% of townhouses and 97% of apartments).
2. Modelled dwelling completions (the remaining building approvals not selected in the BACS sample), which are allocated to the SA2 from the building approval and the probability of completion each quarter contributes to the small area estimate.

The small area completion estimates are calibrated to the state level BACS completion estimates. The small area estimates may then be adjusted:

• to include other additions to dwelling stock such as change in building use (i.e. a short-term holiday apartment to a long-term residential dwelling)
• to account for timing differences between Census and building activity data (i.e. a staged building job will be completed in building activity once all stages are completed, whereas some stages may have been completed before the Census)
• based on coherence with Census data

### Modelled small area estimates

The probability of completion for the modelled dwelling completions is estimated using a log-logistic accelerated failure time model with random effects. The following predictor variables available in the building approval data were selected based on the strength of their relationship to the outcome variable (i.e. dwelling completion):

• State
• Type of building
• Value of approval
• Approval date in 2021 or later (these approvals have experienced higher than usual completion times)

This method produces completion probabilities for each quarter that we expect the dwelling could be completed in (i.e. a dwelling may have a 20% probability of completion in the quarter it was approved, a 30% probability of completion the following quarter etc), which are summed by quarter to create the small area estimate of dwellings completed.

### Quality of modelled small area estimates

The errors associated with the modelled estimates for small areas fall into four categories: sampling error, non-sampling error, modelling error, and prediction error.

Since the estimates for building activity are based on a sample of approved building jobs, they are subject to sampling error; that is, they may differ from the figures that would have been obtained if information for all approved jobs for the relevant period had been included in the survey. Relative standard errors (RSEs) are available in the Building Activity, Australia publication as a measure of the sampling error in building activity statistics. Non-sampling error refers to inaccuracies that may occur in the source of building approval information, imperfections in reporting by respondents, and errors made in the coding and processing of data.

Modelling error is introduced by model misspecification. This can occur when the choice of model is incorrect, a key predictor variable is left out or an inappropriate predictor variable is included. Therefore, the variables chosen in the models may result in incorrect modelled estimates for certain small areas, particularly small areas that do not follow the typical associations between the available predictor variables and the dwelling completions being modelled. The chosen model has been tested against a range of possible alternative models; however, it is the most suitable model subject to available data at the time.

A strong model does not guarantee statistically accurate modelled estimates. Prediction error is a measure of the statistical accuracy of the predictions made to produce the modelled small area estimates. The relative root mean squared error (RRMSE) provides an indication of the deviation of the modelled estimate from the true value. The RRMSE is primarily a measure of prediction error, but in its calculation it also inherits some aspects of modelling and sampling error. As a general rule of thumb, estimates with RRMSEs less than 25% are considered reliable for most purposes, estimates with RRMSEs between 25% and 50% should be used with caution and estimates with RRMSEs greater than 50% are considered too unreliable for general use.

The table below shows the median RRMSEs of the quarterly SA2 dwelling completion estimates, by the size of the estimate. Estimates with higher values (i.e. more dwellings completed) generally have lower RRMSEs. Estimates with very low values (<11 dwellings completed) should preferably be analysed at a broader level (e.g. across multiple quarters or small areas), as these larger aggregated estimates will generally have lower RRMSEs.

Median SA2 RRMSEs by type of building, June 2016 quarter to June 2022 quarter
Number of dwelling completionsHouseTownhouseApartmentTotal building
1 to 1046%>50%>50%>50%
11 to 2024%34%5%24%
21 to 5016%17%2%16%
51 to 10011%4%1%10%
>1007%1%0%3%

## Removals from dwelling stock

Estimates of dwelling stock removals are compiled from:

• Dwelling demolitions – dwellings demolished following a demolition approval (in all states and territories except SA) or electricity abolishment notification (in SA only)
• Unplanned stock losses – dwelling losses resulting from natural disasters

### Dwelling demolitions

A dwelling demolition is defined as the complete and intentional dismantling of a dwelling, such that none of the structure remains on site.

Estimates of dwelling demolitions are modelled based on demolition approvals from local councils or other approving authorities (in all states and territories except South Australia) or electricity abolishment notifications (in South Australia only).

Demolitions approvals are sourced from the Building Approvals collection, which capture dwellings approved by a local council or other approving authority to be fully demolished. Within South Australia, approval to demolish a dwelling is only required where the dwelling falls within a historic zone or heritage area. Demolition estimates in South Australia are based on electricity abolishments data sourced from SA Power Networks (SAPN). Abolishment refers to the permanent disconnection of power supply to a premises, a process that is required before dwelling demolition can proceed.

The probability of a demolition going ahead following approval or electricity abolishment notification (in South Australia only) is modelled using state-specific models based on the distribution of approval or electricity abolishment notification to demolition time frames. The model is based on a sample of demolition outcomes, which were observed using aerial imagery from Nearmap Australia Pty Ltd. This provides an approximation of the demolition date, although the frequency of Nearmap image captures will affect the accuracy of the observed demolition date.

This method produces probabilities for each quarter that we expect the dwelling could be demolished in (i.e. a dwelling may have a 20% probability of demolition in the quarter it was approved, a 30% probability of demolition the following quarter etc), which are summed by quarter to create the small area estimate of dwelling demolitions.

The estimates of dwelling demolitions are subject to sampling error; that is, they may differ from the figures that would have been obtained if all demolition approvals and electricity abolishments had been observed using aerial imagery. Non-sampling error may also arise due to inadequacies in the source of building approval or electricity abolishment information. The estimates are also subject to modelling error and prediction error as outlined in the 'additions to dwelling stock' section above, however measures of these errors are not available for dwelling demolitions.

### Unplanned stock losses

Unplanned stock losses refer to dwellings that are unintentionally and permanently destroyed. This includes dwellings destroyed in natural disasters such as bushfires, floods and cyclones. These types of removals are not accounted for by the Building Approvals collection in a timely way (a decision to seek demolition or building approval is often well after the disaster has occurred). Temporary removals from stock where dwellings may be uninhabitable for a period of time following a natural disaster are not in scope.

Estimates of unplanned stock losses have been compiled based on currently available data from the National Recovery and Resilience Agency (NRRA), which covers the 2019-2020 bushfire season only. This data includes total counts of dwellings destroyed by LGA across the bushfire season. These counts were allocated to SA2s based on the proportion of exposed dwellings in each mesh block within an LGA. This was calculated by multiplying the proportion of the area that was affected by bushfires (based on bushfire boundaries sourced from the NRRA) by the 2016 Census dwelling count. This was then multiplied by the number of destroyed dwellings in the LGA. The estimate of dwellings destroyed in each mesh block was then aggregated to the corresponding SA2. The counts were manually allocated to the appropriate quarters using various news sources.

A collection of the number of unplanned stock losses is not currently available in a nationally consistent and timely manner. The ABS is continuing to investigate alternatives to estimate unplanned stock losses where direct counts are not available (i.e. based on the location of dwellings and hazard exposure information).

## Confidentiality

The ABS collects statistical information under the authority of the Census and Statistics Act, 1905. This requires that statistical output shall not be published or disseminated in a manner that is likely to enable the identification of a particular person or organisation.

To guard against identification or disclosure of confidential information, sensitive data cells may be subject to small random adjustments.

## Data release

The files under the 'Data downloads' heading contain data for small geographic areas in Excel and CSV format (contained within .zip file), as so:

 Geographic classification Statistical Area Level 2 (SA2), Statistical Area Level 3 (SA3), Statistical Area Level 4 (SA4), Greater Capital City Statistical Area, State Time period Quarterly Type of Building Total dwellings Data items Estimated dwelling stock (number)
 Geographic classification Statistical Area Level 2 (SA2) Time period Quarterly Type of Building Houses, Townhouses, Apartments, Total dwellings Data items Estimated dwelling stock (number), Additions (number), Removals (number)

## Abbreviations

### Show all

Abbreviations
AbbreviationDefinition
ABSAustralian Bureau of Statistics
ACTAustralian Capital Territory
ASGSAustralian Statistical Geography Standard
Aust.Australia
BACSBuilding Activity Survey
BAPSBuilding Approvals
FCBFunctional Classification of Buildings
no.number
NSWNew South Wales
NTNorthern Territory
qtrquarter
QldQueensland
RRMSErelative root mean squared error
SASouth Australia
SA2Statistical Area Level 2
SA3Statistical Area Level 3
SA4Statistical Area Level 4
STRDDwelling Structure
Tas.Tasmania
Vic.Victoria
WAWestern Australia

## Glossary

### Show all

The number of dwellings added to stock. Dwellings can be added to stock by construction work (see 'dwelling completions') or changes in building use (i.e. a short-term holiday apartment to a long-term residential dwelling).

Census
The complete enumeration of a specific population at a point in time (as opposed to a survey, which enumerates a sample of the population). When the word is capitalised, "Census" refers to the national Census of Population and Housing. The Census is run by the ABS every five years and aims to count every person and dwelling in Australia on Census night.

Dwelling
A dwelling is a self-contained suite of rooms, including cooking and bathing facilities, intended for long-term residential use. A dwelling may comprise part of a building or the whole of a building. Regardless of whether they are self-contained or not, rooms within buildings offering institutional care (e.g. hospitals) or temporary accommodation (e.g. motels, hostels and holiday apartments) are not defined as dwellings.

Dwelling completions
Dwelling completions include construction of new dwellings, dwellings created as a result of alterations/additions to existing buildings, or through conversion of a non-residential building to a residential building. A dwelling is completed when building activity has progressed to the stage where the building can fulfil its intended function.

Dwelling demolition
The complete and intentional dismantling of a dwelling, such that none of the structure remains on site.

Apartments
Blocks of dwellings that don't have their own private grounds and usually sharing a common entrance, foyer or stairwell, including attached granny flats.

Greater Capital City Statistical Area (GCCSA)
Represent the socioeconomic area of each of the eight state and territory capital cities. These boundaries are built from aggregations of whole Statistical Areas Level 4. GCCSA boundaries represent a broad socioeconomic definition of each capital city, containing not only the urban area of the capital city, but also surrounding and non-urban areas where much of the population has strong links to the capital city, for example through commuting to work.

House
A detached building primarily used for long-term residential purposes consisting of one dwelling unit. Includes detached residences associated with a non-residential building, and kit and transportable homes and detached granny flats.

Non-residential building
A non-residential building is a building primarily intended for purposes other than long-term residential purposes. Note that, on occasions, one or more dwelling units (such as a caretaker’s flat) may be created through non-residential building activity. These dwellings are included in the total type of building category only.

Relative Root Mean Square Error (RRMSE)
The RRMSE is an estimate of accuracy for the modelled values, based on how well the model performs in comparison with known historical data.

Removals
The number of dwellings removed from stock. Dwellings can be removed from stock by demolition work (see 'dwelling demolition') or natural disasters. Temporary removals from stock where dwellings may be uninhabitable for a period of time following a natural disaster are not in scope.

Statistical Area Level 2 (SA2)
A medium-sized general purpose area defined in the Australian Statistical Geography Standard built from whole SA1s. Their purpose is to represent a community that interacts together socially and economically. SA2s are based on officially gazetted suburbs and localities. In urban areas, SA2s largely conform to one or more whole suburbs, while in rural areas they generally define the functional zone of a regional centre. SA2s generally have a population range of 3,000 to 25,000 people, and an average population of about 10,000 people. There are approximately 2,300 SA2s and they cover the whole of Australia without gaps or overlaps.

Statistical Area Level 3 (SA3)
An area defined in the Australian Statistical Geography Standard built up from SA2s to provide a regional breakdown of Australia. SA3s aim to create a standard framework for the analysis of ABS data at the regional level through clustering groups of whole SA2s that have similar regional characteristics. Their boundaries reflect a combination of widely recognised informal regions as well as existing administrative regions such as State Government Regions in rural areas and Local Government Areas in urban areas. SA3s generally range in population from 30,000 to 130,000 people. There are around 360 SA3s and they cover the whole of Australia without gaps or overlaps.

Statistical Area Level 4 (SA4)
An area defined in the Australian Statistical Geography Standard designed for the output of labour force data and to reflect labour markets. In rural areas SA4s generally represent aggregations of multiple small labour markets with socioeconomic connections or similar industry characteristics. Large regional city labour markets are generally defined by a single SA4. Within major metropolitan labour markets SA4s represent sub-labour markets. SA4s are built from whole SA3s. They generally have a population of over 100,000 people to enable accurate labour force survey data to be generated. There are 107 SA4s and they cover the whole of Australia without gaps or overlaps.

Townhouse
Dwellings attached in some structural way to one or more other dwellings, with their own private grounds with no other dwellings above or below.