The fundamental difference
In new build, the building is the outcome of the project. It does not exist beforehand, and every element is chosen, sized and checked on drawings.
In renovation, the building is the input. It exists, it has a history, it has been altered, and part of what it contains is not visible.
The consequence is asymmetric: in new build, uncertainty bears on what will be done; in renovation, it also bears on what is already there.
That is why new-build methods do not transpose as they stand, even where the techniques used are identical.
The five consequences
| Area | In new build | In renovation |
|---|---|---|
| Survey | unnecessary | indispensable preliminary |
| Measurement | determined quantities | partly estimated quantities |
| Budget | a firm price is conceivable | a contingency is necessary |
| Contract | defined object | partly unknown object |
| Programme | predictable sequence | probable interruptions |
Each of those lines is the subject of an article in this guide, because none is solved by goodwill.
What it changes about sequence
One phase exists in renovation and not in new build. It consists in discovering the existing fabric.
The phase of getting to know the building precedes design. It comprises regulatory surveys, measured surveys and, as the case may be, investigative openings.
It has its own cost and duration, often underestimated because they precede visible construction.
Economising on it is the commonest false economy. What is not investigated beforehand is discovered during the works, when any change costs most.
That phase is covered in the branch on surveys and preliminaries. The obligations are set out there.
What it changes about the contract
A renovation contract must say what a construction contract need not. It must provide for the fate of discoveries.
How a discovery is handled, that is an unforeseen element appearing during the works.
How a variation is decided and priced, since they are inevitable rather than exceptional.
Who bears an interruption, where a project stops pending a decision or a third-party intervention.
Those stipulations are not excessive caution, they answer a structural feature of the exercise, covered in the branch on the renovation contract.
The construction contract in general belongs to another guide, on contracts and liability. The liability regimes are compared there.
What it changes about budget
A difference in kind, not in degree.
In new build, a budget is exceeded through choices. Changes of brief, upgrades, late decisions.
In renovation, a budget is also exceeded through discoveries, independently of any decision by the client.
A price announced without a contingency is therefore misleading, not because it is dishonest, but because it covers a scope that is not fully known.
This is covered in the article on pricing with a contingency. The method is set out there.
The case of technical requirements
A useful point for situating this guide against the applicable rules. The technical rules for new build do not apply as they stand.
Technical requirements are not the same in new build and renovation, and they are sometimes triggered when a threshold of scale is crossed.
That question belongs to building regulations, covered in the corresponding guide on new, existing and refurbishment, which explains how each field draws the boundary.
This guide covers the practical consequences: what to know, what to write, what to provide for.
What distinguishes a well-prepared renovation
Four markers, observable before the first blow. They signal a project with high uncertainty.
The building has been investigated, not merely visited.
The budget carries an explicit contingency, quantified and discussed, rather than an implicit margin.
The contract provides for handling discoveries, before a discovery occurs.
The programme includes decision points, at the moments when the building will be open and choices must be made quickly.
A renovation meeting those four markers does not have fewer surprises. It absorbs them better.
The articles in this branch
The article on structural unknowns covers what is not visible before opening up. The consequences are set out there.
The article on measurement in renovation covers uncertain quantities. Its specific features are explained there.
The article on pricing with a contingency covers the provision. The method is given there step by step.
This article sets out principles of professional method and serves as orientation. It does not replace technical advice or assessment of the individual case.