In short: how to catch a swap
A material swap is caught by checking: the reference, series, grade and packaging labels must match what is written in the approved specification, and that check happens on delivery, before installation. Once the tiles are down or the wall is closed, proving the difference is far harder.
Two materials of different grades look almost identical in a photo or a small sample. The difference shows a year later: second-grade tiles with shade variation, parquet that cannot be sanded, cable that runs warm. That is what a swap relies on. If a change is needed for a legitimate reason, such as a discontinued series, that is a separate matter, covered in our article on substituting a material without losing the design intent.
Where swaps happen most
Hidden items: water and waste pipes, cable size, waterproofing in showers and on terraces, tile adhesive, insulation, plasterboard framing. You will never see these again, and the cost of getting them wrong is high: leaks, overheating, cracks.
Visible but hard to tell apart: tile grade and calibre, the wear-layer thickness of engineered parquet, the tap body (brass or alloy), furniture hardware such as hinges and runners, paint from the same brand but a cheaper line. Countertop stone belongs here too: a slab from another quarry or with different veining looks almost the same in a photo, but not next to the approved sample.
A specification with references is the basis
Without a specification there is nothing to check against. 'Grey porcelain tile 60×120' allows any tile of that size. For each material, record the manufacturer, series, reference, grade and size; for parquet, the wear-layer thickness; for cable, the cross-section; for waterproofing, the system. Sign the samples and keep them on site or at home.
The specification should be an annex to the contract, and any change to it made in writing, with the reason and price effect, before installation. A good contractor proposes this way of working unprompted. If you are told 'why all this paperwork, we will do it right anyway', that is exactly when the paperwork matters most.
Checking deliveries
When material arrives, check three things against each other: the delivery note, the packaging label and the specification. Tile boxes show the series, grade, calibre and batch number. All boxes for one area should come from the same batch, or you will get shade variation. Parquet packaging states the board construction. Cable carries its cross-section printed along its whole length.
Ask for packaging and labels to be kept until the stage is finished. Open one box and put a piece next to the signed sample in daylight. If you cannot visit the site, have the person in charge send photos of the labels with every delivery. It takes a minute and removes most doubts.
Concealed work: before it is closed
Pipes, cables, waterproofing and insulation are checked while they are exposed. Ask for photos with the markings in frame: the pipe with its print, the cable with its size, the membrane roll next to the laid layer. For showers and terraces, add a photo of the water test.
Ideally, closing up each piece of concealed work is its own point in the programme: the person in charge shows it, you or your technician look, and only then does the crew close it. On an Eixample flat renovation that usually means bathroom and kitchen pipework, the electrics throughout, and the floors if there is underfloor heating.
If you have already found a swap
Do not accept the stage as finished. Photograph the installed material, the label if it still exists, and the approved sample next to it. Compare against the specification and write to the contractor: what was approved, what was installed and how they differ. Ask for an explanation and a proposal to put it right.
Staged payment is your main tool here: while the mismatch is open, payment for that stage can be held back if the contract provides for it. Do not accept a 'discount' for a worse material when it affects safety or watertightness, as with cable or waterproofing: those get replaced, not compensated.