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What is a Provisional Sum?

A Provisional Sum is an estimated allowance in a building contract for work whose full scope or cost cannot be determined at contract time. The most common example is site costs — soil conditions can't be fully known until testing is complete.

Can Provisional Sums increase after signing?

Yes — by definition. If the actual cost of the works covered by a PS exceeds the allowance, you pay the difference. If it's less, you receive a credit.

How can I protect myself from Provisional Sum blowouts?

Request as many PS items to be fixed or converted to Prime Cost items before signing. For site costs, obtain a geotechnical report before contracting. Ask your builder for historical PS variation data.

What is a realistic site cost Provisional Sum?

This depends heavily on your soil classification. An M class flat site may have genuine site costs of $8,000–$15,000. Builders sometimes set PS allowances of $5,000 — which is unrealistically low for most sites.

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Building Contracts & Costs

Provisional Sums Explained

What Provisional Sums are, why they vary, and why they're one of the biggest risks in a new home building contract.

5 min·Compare My Build — Independent Resource· Updated June 2025

Introduction

Provisional Sums (PS) are estimated cost allowances in your building contract for items where the full scope or cost is unknown at the time of signing. They are a normal part of residential building contracts — but they are also one of the most significant sources of budget blowout. Understanding how they work is essential before you sign.

Common Provisional Sum Items

PS Item	Typical PS Allowance	Typical Actual Cost
Site costs	$5,000 – $10,000	$10,000 – $40,000+
Rock excavation	$3,000 – $5,000	$5,000 – $25,000+
Retaining walls	$5,000	$8,000 – $30,000+
Service connections	$3,000 – $5,000	$5,000 – $20,000 (rural)
Landscaping	$0 – $5,000	$15,000 – $50,000+
Pool	Not applicable	$50,000 – $150,000+

Watch Out

Site cost Provisional Sums are the most dangerous. A builder may set a $5,000 PS for site costs when the realistic cost on a typical suburban block is $15,000–$30,000. The difference comes out of your pocket after signing.

Common Builder Inclusions

›Review every PS item in your contract and challenge unrealistic allowances
›Order a geotechnical report before signing to get a more accurate site cost estimate
›Ask your builder: 'What is the average PS variation in your last 20 completed homes?'
›Try to convert PS items to fixed prices wherever possible
›Always budget a 15–20% contingency above total PS allowances