How to replace a material without losses
First, define why the original material was chosen: tone, format, texture, performance in use. Then compare candidates with physical samples in the same room, check which works change along with the material, work out the full cost of the swap and approve a specific product code in writing. Five steps, and none can be skipped.
Substitutions happen in renovations more often than anyone would like: a collection is discontinued, a batch arrives in a different shade, the lead time stretches to three months, or the budget has to be trimmed. A substitution in itself is not the problem. The problem is when it is decided from a catalogue photo and the consequences show up during installation. If you suspect a material was swapped for a cheaper one without your knowledge, that is a different situation, covered in its own article.
Why the original was chosen
Write down the non-negotiable properties in a sentence or two. For a herringbone parquet like the one in the Urgel flat in the Eixample, for example: the wood species and tone, the length-to-width ratio of the plank that defines the pattern, the type of finish (oil or lacquer) and the overall thickness that sets the floor level at the thresholds. If any of these changes, it is a different floor, however similar it looks in a photo.
For bathroom tiles the fixed properties might be the format and grout width; for a worktop, stain resistance and the edge profile; for paint, the sheen level. This short list becomes the test: a candidate that fails it is out immediately, before price is even discussed.
Compare samples in the room
Screens lie, and so do catalogues. The shade of wood, stone or tile depends on light, and a north-facing Eixample room overlooking the courtyard shows a material very differently from a showroom. Ask for samples big enough to judge, several tiles to see the variation, several planks of wood, and place them next to what is staying: the adjoining floor, the kitchen fronts, the worktop.
Look at them by day and in the evening, under the fittings that will actually be installed. Photograph the samples together in one frame and keep it: a month later it will be the only objective record of what you approved.
Check the knock-on changes
A different thickness changes the junctions: the transition between parquet and tile, the door undercuts, the level at a tray-less shower. A different format changes the layout, the number of cuts and the waste percentage, and sometimes whether the joints line up with a niche or the edge of the worktop. A heavier worktop may need the kitchen carcasses reinforced.
Installation changes too: another adhesive, another underlay, compatibility with underfloor heating. So does maintenance: porous stone needs sealing, and matt porcelain on a kitchen floor holds dirt differently from polished. These questions go to the installer and the supplier before the decision, not after. Best of all, with three people together: you, the site lead and whoever will lay it.
Work out the full cost of the swap
The price difference per square metre almost never equals the difference in cost. A cheaper large-format porcelain tile may need a levelling system and a more experienced tiler and produce more waste, ending up dearer than the original. A material delivered in two months costs idle crew days or a postponed stage.
Ask the site lead for a short calculation: material, labour, accessories (trims, skirting, adhesive), waste and programme impact. The decision is made on the final figure, not the catalogue price. If the swap is meant to save money, this calculation often shows it is better to save somewhere else.
Approve a specific product code
Agreeing to "something similar" is not enough. The approval records the manufacturer, collection, product code, colour and, for tile and stone, the batch number, plus any changes to cost and timing. The accepted sample is signed and kept on site so there is something to compare against at handover.
Then the specification and every related document are updated: the supplier order, the layout drawing, the installer's brief. Take extra care when two shades have similar names: a one-digit error in a product code is discovered when half the room is already laid. On our projects one person is responsible for the site, and it is their job to make sure the shop, the crew and the client are all working from the same code.