---
title: "How We Calculate Duplex Cost & Feasibility | DuplexCost"
description: "Learn about the exact mathematical cost models, multipliers, and financial formulas used by DuplexCost to calculate turnkey duplex feasibility in Australia."
source: DuplexCost
sourceUrl: https://duplexcost.com/how-we-calculate/
lastUpdated: 2026-07-01
citationUrl: https://duplexcost.com/how-we-calculate/
---

# How We Calculate

Learn about the exact mathematical cost models, multipliers, and financial formulas used by DuplexCost to calculate turnkey duplex feasibility in Australia.

An inside look at our mathematical costing algorithms and ROI formulas.

The base construction cost represents the core building contract. It is calculated by taking the total building footprint (both sides combined) and applying our city and quality-adjusted rates, storey factor, and configuration type (attached vs detached):

To transition a suburban site from an old single dwelling to a brand-new duplex, several civil works surcharges are calculated:

These represent the soft costs traditionally omitted by volume builders:

To determine if a duplex project is financially viable, the calculator models:

## 1. Base Construction Cost Formula

## 2. Infill Civil & Site Preparations

## 3. Subdivision, Levies, & Connections

## 4. Feasibility, GRV, & Rent Yields

#### Guides

#### Company

## Key Points

- Demolition of Old Home: $22,000 baseline (Sydney single storey), adjusted by city multipliers (e.g. Sydney: 1.20, Melbourne: 1.10). A flat $10,000 surcharge is automatically added if the home was built pre-1980 (asbestos risk).
- Slope Factor: Surcharges of flat ($0), slight ($15k), moderate ($45k), or steep ($85k) are applied to account for engineered structural retaining walls, drop-edge beams, and stepped slabs.
- Design & Survey package: $25,000 (standard), $45,000 (premium), and $85,000 (luxury) covering contour surveys, geotechnical soil tests, engineering, and town planning fees.
- Legal land subdivision titling: Torrens Title ($18,000) or Strata Title ($10,000).
- Council Developer Contributions (S7.11/7.12): Surcharged based on city (Sydney: $25k, Melbourne: $18k, Brisbane: $15k, Perth: $12k) per extra dwelling.
- Holding Costs: Sourced from city median weekly rents over a 78-week (18 months) development phase.
- Gross Realization Value (GRV): Sourced as $2 \times [(Current Land Value \times 0.70) + (TDC / 2 \times 1.12)]$, representing the combined market value of the completed duplex units.
- Manufactured Equity: Gross Realization Value minus Total Project Cost (Raw Land Value + TDC + Holding Costs).
- Gross Rental Yields: Expressed on development capital (excl. land) and total capital invested (including land).
- Cost to Build a Duplex

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*Source: [DuplexCost](https://duplexcost.com/how-we-calculate/)*