Water is most of the cup and the variable people change last

Coffee is over 98 percent water and it is the thing almost nobody adjusts. Changing it fixed problems I had been chasing with grind size for a year.

What water does

• Minerals extract flavour. Distilled or fully softened water makes flat, hollow coffee no matter what else you do, because there is nothing to carry the extraction.
• Too hard and it goes chalky and dull, and it destroys equipment through scale.
• Alkalinity flattens acidity. High-alkalinity water neutralises exactly the brightness that makes a good coffee taste like anything, which is why the same beans taste flat at one house and excellent at another.

How to tell it is your water

• The same beans and the same method taste noticeably different somewhere else.
• Everything tastes muted and dull regardless of grind or ratio.
• Scale builds quickly in the kettle, which tells you it is hard.

What to do, cheapest first

• Try bottled water for a few brews as a test, not as a permanent solution. Look for something in a middle range of mineral content rather than the softest available. If the coffee improves markedly, you have your answer for a few coins.
• A filter jug helps with chlorine and some hardness, and is the practical middle option.
• Mineral sachets or concentrates added to distilled water for people who want it consistent. Repeatable and the most controlled option.

Two practical notes
Chlorinated tap water is worth filtering regardless, because chlorine flavour survives brewing. And for any machine with a boiler, water choice is also a maintenance decision, since hard water is what eventually kills them.

The honest boundary: if your tap water is decent, filtering the chlorine is most of the benefit and the rest is refinement.