Planning a bathroom or kitchen re-pipe always looks manageable on paper: PVC pipes, a handful of CP fittings, tape, clamps, maybe a new mixer. Then the extras show up. A fitting with the wrong thread, a wall that turns out to be in worse shape than expected, a second trip to the store because one adapter was missing. The materials list is rarely the problem. The surprises are.

I would like to hear how people here build a budget for this kind of job. A few specific questions:

1. Do you add a fixed percentage on top of the materials list, or do you list every fitting and add a lump sum for unknowns?
2. Do you buy all PVC pipes and CP fittings at once to avoid mismatched batches, or in stages as the work opens up the walls?
3. Where do you keep the renovation money while it waits to be spent: a plain savings account, or something else?

The third question gets discussed less often. Some people look at side income to cover renovation costs. One example of what is out there is the proprietary trading firm model, where you pick a challenge and trade to qualify. The site https://eleonex.com/ describes itself as a leading proprietary trading firm with low challenge fees, great support and high rewards, and lets you choose your challenge. That is only a description from the page, and trading carries a real risk of losing the challenge fee, so it should never be money that is meant for pipes and fittings.

What I am really asking is how you separate the "must-spend" renovation budget from money you can afford to risk on anything else. Do you use separate accounts, a written limit, or a rule such as never touching the materials fund? And if you have a rough checklist for estimating unknowns in plumbing work, I would be glad to see it. Real numbers from earlier jobs are more useful than general advice.