What to do first
Stop approving new work until the numbers are clear. Split the overrun by cause, ask for a written price for each item, and only then decide: pay, simplify or postpone. It takes a few days, and in the meantime the crew can carry on with the work already agreed.
What you should not do is unilaterally stop paying for stages that have already been completed and signed off. That turns a dispute about extras into a dispute about breach of contract, which takes longer and costs more to resolve. The conversation about new amounts is kept separate from paying for work already done.
Split the overrun into three causes
The first is your own changes: different tiles, an extra socket, moving a door, a built-in wardrobe that was not in the design. These are your decisions and they get paid for, but at a price agreed in advance.
The second is technical surprises that could not be seen before strip-out. In an Eixample flat, a damaged floor structure may turn up under the old tiles; in a sixties building, lead pipes or wiring with no earth. An honest contractor flags these as risks from the start, and some of them are covered by the contingency, if the quote had one.
The third is the contractor's own underestimate: fewer square metres of tiling than wall area, a screed they forgot to break out, debris removal left out. That risk is not yours. Calmly point out the gap against the quote and contract, and discuss who pays for it. Once the overrun is split this way, the conversation is about facts rather than feelings.
Check against the quote and contract
Open the contract and look for three things. What type of quote you have: a fixed price for an agreed scope, or payment by actual measurement. In the first case, quantities growing without design changes are the contractor's problem; in the second, it is a normal part of the process, but then you should be shown the measurement schedule. The difference between the two models is explained in a separate article.
Next, whether the contract sets out a procedure for approving changes and a contingency allowance. If the procedure exists and the work was done without written approval, your position is strong. If it does not exist, add it now as an addendum, before the next extra.
Every extra in writing, before it is done
For each additional item, ask for a short document: what exactly is being done, in what quantity, at what unit price, and how it affects the programme. The unit price should come from the original quote where possible: if wall tiling was priced per square metre there, the new metres should not cost more without an explanation. You sign, the work goes ahead. A verbal "yes, go ahead" in the hallway is the most common source of disputes at the end of a renovation.
There is one exception: urgent technical situations, such as a leak from a freshly exposed pipe. It is fixed immediately, but on the same day photos and a written message describing it are sent. The price follows later, but the fact and the date are on record.
Where to cut and where not to
You can cut what is easy to add later: decorative lighting, some furniture, textiles, the interior fit-out of a dressing room that can be ordered six months on. You can simplify the tile layout or format, if it does not affect falls or junctions.
You do not cut what gets closed up for good: building services, waterproofing, acoustic insulation, wiring in walls, floor substrate preparation. Savings there come back within a year as a leak or cracks, and then you pay to rip out finished work. If you have to choose, it is better to move in without some of the furniture than with walls that are new on the outside and old inside.
When to get a second opinion
If the overrun is large and involves the structure or building services, along the lines of "the beam needs strengthening" or "the whole riser must go", it is sensible to bring in an independent technician, such as an arquitecto técnico, before approving it. They look at what has been opened up, not at the quote, and tell you whether the work is needed at that scale. The visit costs money, but far less than an unnecessary floor reinforcement.
For small extras a second opinion rarely pays off: it is simpler to compare the unit price with the original quote.
How we reduce the risk of overruns
We prepare the quote only after the site visit and measurements, and it shows what is included and what is not. Payment follows the contract in stages, so you pay for what has been done, not what has been promised. Any change is priced and agreed in writing first. One person is responsible for the site, and all extras go through them rather than through individual tradespeople.
In an old building surprises cannot be ruled out entirely: what is inside a wall only becomes visible once it is opened. But every surprise can be turned into a clear document with a price, rather than an unexpected figure at the end.