How much contingency do you really need for a renovation?

04 Oct 2026, 12:00 by Shay Lally, https://www.thejournal.ie/author/shay-lally/ · TheJournal.ie

IN THE BIG Reno Clinic series, Houses to Restore QS Shay Lally provides answers and solutions to the big (and little, but important) questions you may have about making the most of your home.

Send your question for Shay to reno@thejournal.ie

This week’s question:

What’s a realistic percentage of the overall cost of a renovation for my contingency fund, and what kind of things eat into it?

Shay says:

Short answer: 10–20%. Here’s why, and where that money actually goes…

Firstly, what is contingency? Well, every renovation job will have a price agreed at the start, but it’s advisable that every homeowner also separately has some money, a percentage of that cost in their back pocket, for the unexpected and unforeseen costs that might come your way during the works. You won’t always need to use your contingency, but if it comes up, you’ll be glad you have it.

Every older house has unknowns that only become apparent after it’s been gutted — rotten joists, defective pipework, structural issues and issues with subfloors. You can’t see them until the house is opened up. But after years as a QS and running a refurbishment company, I can tell you the same issues come up again and again on the same house types.

A few I see constantly: in Victorian houses with a return, the ceiling joists in the return nearly always need doubling up. In ex-corporation houses, the original concrete subfloor has a bitumen damp-proof layer that degrades over time, rotting the timber floorboards above it — once that happens, the floor needs to come out and OSB board and levelling compound need to be fitted before you can install your new floor covering. In the worst cases, you’re also treating rising damp – but generally rising damp is apparent before you start. On bigger jobs, stripping back brick walls sometimes reveals there were never concrete lintels above the doors in the first place or the lintels above the windows are pieces of rotten timber — or that the house needs underpinning because the foundation is defective

Where you sit in that 10–20% range depends on the age and condition of the house. An apartment I renovated that was built in 2002 that just needed a cosmetic refresh threw up no surprises at all. As a result I didn’t touch the contingency. The 1890 house I’m renovating now has caught me out once so far: stripping the wallpaper revealed the walls were in worse condition than I expected even with my background.

The trap: a partial renovation is often riskier than a full one. By gutting a house completely and replacing everything, you’re removing a lot of the unknowns, because everything’s being replaced anyway. Renovating a section of house can throw up more surprises. For example opening up a kitchen and dining room into an open-plan space can expose electric, heating or plumbing issues nobody could have foreseen. Whoever’s doing the work then has to decide where to draw the line — and most will suggest doing a bit more work than strictly necessary to correct the issue, because fixing it properly now prevents the problem resurfacing later.

Scope creep is the other thing that eats into contingency. That’s when you decide mid-project to do extra work because it makes more sense to do it all in one go. My own attic is a case in point — I’d planned to just replace the stairs and leave the rest, but once I had the keys I committed to renovating the whole room, and pushed solar panels and repointing the front of the house out to next year instead in order to make my budget work. There’s nothing wrong with my choice, but it’s exactly the kind of decision that draws down your contingency rather than an unforeseen problem.