Cost Forecasting Example — City of Greening

Overview

This worked example shows how a council can build a portfolio-scale cost forecast for an urban tree program and translate it into Long-Term Financial Plan inputs. It demonstrates cohort-based forecasting, maturity creep, portfolio cashflow modelling and the distinction between CAPEX, OPEX, renewal and monitoring expenditure.

The principal example uses the fictional City of Greening, a medium-sized metropolitan council with approximately 180,000 residents. The council is pursuing canopy expansion from 23% to 30% over a 25-year planning horizon. The figures are indicative and should be replaced with local inventory data, unit rates, contract prices and finance-team categories.

Use with Steps 7 and 8

A portfolio-scale worked example

Step 7 explains the cost-forecasting methods and Step 8 explains financial strategy and LTFP integration. This resource shows how those ideas can be worked through at portfolio scale, using urban trees as the main example and a constructed wetland as a hybrid GI example.

Planning Model

Work this with your own data

This is a worked illustration — the figures shown are illustrative. To build a cost forecast for your own portfolio, with your own rates and assumptions, work Steps 5 to 8 in the online Planning Model.

Method

What this example shows

Method	What it does in this example	Output
Method 1 — cohort-based forecasting	Groups urban trees into establishment, young, semi-mature and mature cohorts so costs can change as the portfolio ages.	Maturity distribution, canopy trajectory and OPEX requirement.
Method 2 — component-based costing	Builds costs from asset components. This is demonstrated later for hybrid GI assets such as constructed wetlands, where engineered, vegetation, sediment and monitoring components behave differently.	Component cost schedule, renewal timing and OPEX/CAPEX split.
Method 3 — portfolio cashflow modelling	Combines CAPEX, OPEX, renewal and monitoring streams into a year-by-year expenditure profile for Finance review and LTFP integration.	Years-across expenditure profile, funding requirement and scenario input.

How the methods work together

Stage	Question answered	Output	Main users
Method 1 — cohorts	How does the cost profile change as living assets mature?	Maturity distribution, OPEX requirement, canopy/service trajectory.	Asset managers, arborists, urban forest teams.
Method 2 — components	Which elements need maintenance, renewal, replacement or stewardship?	Component cost schedule and renewal timing.	Asset managers, drainage/WSUD teams, Finance.
Method 3 — portfolio cashflow	What expenditure profile must be funded over the LTFP horizon?	Years-across CAPEX/OPEX table, funding gap and scenario input.	Finance, executives, councillors, AMP owners.

1. Worked example — City of Greening urban trees

Baseline assumptions

The baseline assumptions define the physical and financial scale of the model. They are the starting point for the cohort model, OPEX calculation, CAPEX profile and LTFP-ready table.

Parameter	Value / Range	How it is used in the forecast
Land area	150 km²	Context for canopy target and service coverage.
Existing tree stock	≈ 60,000	Opening portfolio size for cohort and OPEX modelling.
Current canopy cover	23%	Baseline service level.
Canopy target	30% by Year 25	Desired service outcome and scenario-testing target.
Average planting cost	$550 per tree	Input to new planting and renewal CAPEX, checked against local program estimates.
Establishment maintenance	$25 per tree p.a. for Years 0–5	OPEX for intensive early care.
Young-tree maintenance	$20 per tree p.a.	OPEX as trees move beyond establishment.
Semi-mature maintenance	$35 per tree p.a.	OPEX for higher pruning, formative maintenance and risk-management needs.
Mature-tree maintenance	$12 per tree p.a.	Routine pruning and inspection cost for established trees.
Annual loss rate	1.5%	Mortality, removals and decline requiring replacement or service-risk response.
Annual renewal rate	1.0%	Replacement program used to smooth renewal demand.
Planting program	3,000 trees p.a. in Years 1–3; 500 p.a. from Year 4 onward	Creates early canopy expansion and later steady-state program demand.
Stewardship / monitoring overhead	$100,000 p.a.	Portfolio-wide program management, data, monitoring and reporting cost.
Planning horizon	25 years	Shows both LTFP horizon and longer lifecycle effects.

Practice note

Rolling renewal smooths the forecast

Councils with inventory data can apply rolling-renewal logic where a percentage of trees is retired and replanted each year. This moderates renewal peaks and produces smoother OPEX and CAPEX curves over the 25-year planning horizon.

2. Build the expenditure profile

Model cohorts, maturity creep and canopy growth

Maturity creep

Living assets move gradually between cost stages

Overlapping maturity cohorts showing establishment, young, semi-mature and mature tree cohorts over time

Overlapping maturity cohorts. Living assets move gradually between lifecycle stages. A portfolio model should allow cohorts to overlap rather than assuming that all trees planted in the same year move to the next cost stage at the same time.

Maturity class	Typical cost behaviour	Forecast implication
Establishment	High early watering, formative care and establishment monitoring.	Early OPEX is intensive but attached to a smaller cohort.
Young	Lower routine maintenance, with continued survival checks and formative pruning.	Costs start to smooth as trees leave establishment.
Semi-mature	Increasing pruning, inspection and risk-management requirements.	OPEX grows as the expanding planting programme matures.
Mature	Routine inspection, pruning and risk works; renewal planning becomes more important.	Long-term OPEX stabilises and renewal CAPEX begins to reappear.

OPEX growth curve showing annual operating expenditure increasing as the tree portfolio matures over the 25-year planning horizon — **OPEX growth driven by maturity creep.** Operating expenditure increases as a larger proportion of the tree portfolio moves into semi-mature and mature lifecycle stages, even after the initial expansion planting program slows.

This annual OPEX profile is the bridge between the cohort model and the long-term portfolio view. It shows why the operating budget keeps rising after the initial planting pulse has slowed: the portfolio is carrying more assets in higher-cost lifecycle stages.

25-year portfolio view

These charts provide the strategic lifecycle view behind the annual LTFP-ready table. They show why the first ten years should not be read in isolation from the longer portfolio cost trajectory.

Portfolio cashflow profile. CAPEX is front-loaded during the expansion program and returns later through renewal, while OPEX grows as the portfolio matures. This is the strategic lifecycle view; the LTFP-ready table converts the first ten years into annual finance categories.

How to read this profile

The annual cashflow profile separates the short-term planting pulse from the longer-term stewardship obligation. CAPEX is highest during the expansion phase, then settles into renewal and replacement. OPEX rises as the portfolio matures because more trees require inspection, pruning, risk management and renewal planning.

The cumulative view below is a different lens. It does not replace the annual forecast used in the LTFP. It helps decision-makers see the long-term funding commitment created by the lifecycle strategy.

Cumulative lifecycle expenditure chart showing cumulative CAPEX, cumulative OPEX and total lifecycle cost over 25 years — **Cumulative lifecycle expenditure.** Cumulative views help explain the long-term funding commitment. They should be used for strategy and scenario comparison, not as a substitute for the annual LTFP-ready profile.

3. Prepare the LTFP-ready expenditure profile

The LTFP-ready table is different from the milestone profile. It shows the first ten years in annual detail, using categories that Finance can map into the council’s budget system.

Scroll sideways to view the full 10-year LTFP-ready profile →

LTFP expenditure category	Typical council budget line	Yr 1	Yr 2	Yr 3	Yr 4	Yr 5	Yr 6	Yr 7	Yr 8	Yr 9	Yr 10
New / growth CAPEX	Capital Works — New Assets / Growth	1,350	1,350	850	450	250	220	200	190	190	190
Renewal CAPEX	Capital Works — Asset Renewal	150	150	150	300	250	230	220	210	210	210
Routine maintenance OPEX	Operating Budget — Urban Forest / Open Space Maintenance	760	800	845	910	980	1,045	1,110	1,175	1,250	1,330
Reactive / risk works OPEX	Operating Budget — Reactive Maintenance / Risk Response	90	95	100	110	120	135	140	145	150	150
Monitoring and stewardship OPEX	Operating Budget — Monitoring / Program Management	100	105	105	100	100	100	100	100	100	100
Total CAPEX ($’000)		1,500	1,500	1,000	750	500	450	420	400	400	400
Total OPEX ($’000)		950	1,000	1,050	1,120	1,200	1,280	1,350	1,420	1,500	1,580
Total expenditure ($’000)		2,450	2,500	2,050	1,870	1,700	1,730	1,770	1,820	1,900	1,980

Practitioner tip

Keep the AMP in base-year dollars

Present cost forecasts in base-year or real dollars within the AMP. The LTFP will usually apply corporate escalation rates when converting to nominal budget values. This keeps the AMP’s service, risk and lifecycle logic clear and avoids confusing real cost growth with inflation.

4. Integrate with Finance systems and reporting

Integration workflow

Step	Action	Why it matters	Output
1	Prepare model outputs in base-year dollars.	Creates a clear AMP cost base before corporate escalation.	Lifecycle cost forecast.
2	Separate new/growth CAPEX, renewal CAPEX, routine OPEX, reactive/risk OPEX and monitoring/stewardship OPEX.	Allows Finance to map the forecast to budget and chart-of-account categories.	Expenditure category schedule.
3	Build the years-across LTFP table.	Matches the structure used in many council LTFP systems.	10-year LTFP-ready profile.
4	Validate categories, timing and cost base with Finance.	Prevents AMP costs from being rejected or reworked later.	Finance-reviewed funding requirement.
5	Compare required expenditure with current LTFP allocations.	Identifies funding gaps and service-potential consequences.	Funding gap and risk statement.
6	Feed the result into Step 8 financial strategy.	Connects lifecycle need to scenarios, funding sources and governance decisions.	Preferred financial strategy.

Corporate systems alignment

System	Alignment approach
Financial Asset Register	Record GI assets where establishment costs are known and can be verified. Confirm financial reporting treatment with Finance and auditors; do not let accounting treatment remove lifecycle cost visibility.
Operational Asset Register / GIS	Maintain location, condition, maturity, ecological and service attributes. Link asset records to lifecycle cost profiles derived from cohort or component methods.
Financial Systems / LTFP	Import time-phased CAPEX and OPEX forecasts in years-across format, aligned with capital works, renewals and recurrent budgets.
Performance and Risk Systems	Link expenditure to service performance, risk and service potential indicators, so the financial strategy can be tested against Step 3 LOS and Step 6 risk settings.
Corporate Reporting Frameworks	Connect GI cost visibility to annual plans, sustainability reporting, community outcomes reporting and internal assurance processes.

5. Apply component costing to hybrid GI

Constructed wetlands, raingardens and other hybrid GI assets often combine engineered elements with living or ecological service components. Method 2 is useful because it separates components by cost behaviour rather than forcing the whole asset into one treatment.

Component type	Examples	Cost behaviour	Typical council category
Engineered structures	Inlets, outlets, pipes, pits, weirs, embankments, liners, access structures.	Often planned as capital works or renewal expenditure, with condition-based renewal cycles.	Capital Works — Stormwater / WSUD Renewal or New Assets.
Earthworks and sediment systems	Sediment forebays, basins, channels, dredging zones.	May be OPEX maintenance for routine cleaning or CAPEX rehabilitation for major desilting and reconstruction. Confirm thresholds with Finance.	Operating Maintenance or Capital Renewal, depending on scale and policy.
Vegetation and habitat	Macrophytes, riparian planting, habitat features, ecological buffers.	Requires establishment, replanting, adaptive maintenance and ecological stewardship.	OPEX — Vegetation Maintenance / Environmental Stewardship; selected establishment works may be CAPEX.
Monitoring and performance verification	Water-quality testing, condition inspections, sediment-depth checks, ecological monitoring.	Recurrent operating cost required to demonstrate treatment performance and service potential.	OPEX — Monitoring / Program Management.

Worked example: constructed wetland lifecycle costs

A council has developed lifecycle cost forecasts for a 1,000 m² constructed wetland. The component model identifies establishment CAPEX, routine OPEX, monitoring/stewardship OPEX and periodic renewal events.

Cost line	Typical council category	Yr 0	Yrs 1–11 p.a.	Yr 12	Yrs 13–24 p.a.	Yr 25
Wetland construction / establishment	Capital Works — New WSUD Asset	$100,000	—	—	—	—
Routine vegetation and litter maintenance	OPEX — WSUD Maintenance	$1,800	$1,800	$1,800	$1,800	$1,800
Monitoring and stewardship	OPEX — Monitoring / Environmental Stewardship	$1,000	$1,000	$1,000	$1,000	$1,000
Major dredging / rehabilitation	Capital Works — WSUD Renewal	—	—	$25,000	—	—
Major renewal	Capital Works — WSUD Renewal	—	—	—	—	$25,000
Total		$102,800	$2,800	$27,800	$2,800	$27,800

Hybrid GI costing

Separate cost behaviour before debating accounting treatment

A hybrid GI asset may include engineered components that behave like traditional infrastructure and living components that require ongoing stewardship to maintain service potential. The cost forecast should separate these components first. Finance and asset teams can then agree whether each cost is treated as CAPEX, renewal, OPEX or stewardship within local accounting and budget policies.

6. Adapt the template across GI asset classes

The same model structure can be adapted to other GI asset classes. The key is to retain the years-across financial output while changing the input logic to reflect the asset’s lifecycle states, components and cost drivers.

Asset type	Lifecycle states or components	Major cost drivers	Forecasting method	Typical finance integration issue
Urban trees	Establishment → Young → Semi-mature → Mature	Watering, pruning, inspections, renewal planting.	Method 1, then Method 3.	OPEX growth from maturity creep can be underestimated.
Constructed wetlands	Vegetation, sediment basin, hydraulic structures, access and monitoring.	Dredging, vegetation management, structural renewal, performance monitoring.	Method 2, then Method 3.	Separate engineered renewal from ecological stewardship.
Raingardens / biofilters	Filter media, plants, inlets/outlets, overflow structures.	Media replacement, sediment removal, replanting, unclogging.	Method 2.	Small assets can be numerous; aggregate by cohort or catchment.
Green roofs / living walls	Membrane, drainage layer, substrate, irrigation, vegetation.	Access, specialist maintenance, membrane renewal and plant replacement.	Method 2.	Some costs may sit with buildings rather than open-space budgets.
Remnant bushland	Vegetation condition, weed pressure, regeneration stage, access/risk assets.	Weed control, ecological burns, fencing, track/access management, monitoring.	Method 1 / hybrid.	Service potential depends on ecological condition, not only capital replacement.
Coastal dunes and foreshores	Dune vegetation, access structures, erosion controls, sand systems.	Storm repair, renourishment, access renewal, adaptive management.	Hybrid.	Climate adaptation costs may not follow traditional renewal cycles.

How this supports Steps 7 and 8

Use this Resource when...	Related Step	How it helps
You need to explain how OPEX grows as living assets mature.	Step 7	Use the maturity-creep and Year 10 OPEX calculation examples.
You need a year-by-year table for Finance.	Step 8	Use the LTFP-ready expenditure profile and finance-category mapping.
You need to cost hybrid GI assets.	Steps 7 and 8	Use the wetland component-costing example to separate engineered components from living-service components.
You need to adapt the method to another GI asset class.	Steps 7, 8 and 9	Use the template adaptation table and update assumptions in later monitoring cycles.

Next planning use

Use the worked forecast in Step 8

Once the cost forecast has been built, use Step 8 to compare it with current LTFP allocations, test funding adequacy and select a financial strategy that sustains service potential over time.

Continue to Step 8 — Financial Strategy →