In short: plan around the period with no bathroom
If the flat has only one bathroom, the job is planned around the stretch when it cannot be used at all: from shut-off and strip-out until the WC and shower work again after waterproofing, tiling and grouting. How long that lasts depends on the scope and the technical waiting times, so go by a programme tied to stages rather than a generic promise of two weeks.
What you can do in advance is choose everything and have it delivered before strip-out, and decide where you will live during those days. The rest depends on the crew and on whether the programme honestly allows for curing and testing.
When exactly the bathroom is out of use
The chain runs: strip-out, first-fix plumbing and electrics, pressure test, waterproofing with its curing time and a leak test, screed with falls, tiling, grouting, then fitting the WC, basin, taps and shower screen. In the first days after strip-out it is sometimes possible to reconnect the WC temporarily overnight, but that depends on the specific setup and the site manager decides.
Waterproofing and screed cannot be cut short: they need their time, and rushing here ends in a leak or cracked tiles. Everything else can be compressed with good preparation.
Where to live during those days
There are three options. Move out: a short let nearby, family or a hotel. With children this is nearly always right. Stay and shower elsewhere, at the gym or the office, using a neighbour's or a café's toilet: that works for one adult for a few days, not for weeks. The third, a temporary toilet in the flat, rarely suits high-end clients in practice.
Compare the Urgel flat in the Eixample, which has two bathrooms: there they can be done one after the other while you live at home. With a single bathroom that option does not exist, and the decision to move out should be made before signing the programme, not on the day of strip-out.
Everything chosen and delivered before strip-out
For a bathroom you cannot live without, an open decision costs more than anywhere else. Before the planned start, specific models must be approved for the WC and concealed frame, taps and their concealed bodies, the drain, the vanity unit and the mirror. The concealed valve bodies are needed at first fix, not at the end.
A collection of photos on your phone is not a confirmed product. You need data sheets with dimensions, confirmation of stock and a delivery date. Ideally tiles, adhesive, grout and sanitaryware are already in the flat or in the contractor's store when strip-out begins.
How to shorten the timeline without losing quality
Three things genuinely speed things up: everything on site before starting, a crew that works every day without breaking off for other jobs, and each trade arriving the day the previous one finishes. The vanity can be assembled in advance in another room, and the shower screen can be ordered from drawings with the measurement confirmed after tiling.
Rapid-drying screeds and adhesives exist, but whether to use them on a particular substrate is for the specialist to decide, not the wish to move back three days early. Do not skip the waterproofing test to gain time: it is the one stage whose mistake you will hear about from the neighbour below.
Partial use before completion
Sometimes the WC and basin can be connected before the shower is finished. The person responsible for the site decides and sets the conditions: what not to use, what not to touch and how long to wait after grouting. Fresh silicone and grout are easily damaged by water and cleaning products in the first few days.
Do not ask for checks to be skipped so you can move back sooner. If you return before completion, agree where the crew leaves its tools and how finished work is protected, so nothing has to be redone.
What to agree with the contractor
Ask for a dedicated line in the programme: the planned dates without a bathroom and the stages they depend on. Agree a plan in case a delivery is late: what you do and what the crew does. On our projects there is one person in charge, and you can deal with them in Russian or Spanish, so the question of when the shower will work has a single addressee.
The return date is adjusted to actual materials and progress. A good contractor flags a delay as soon as they see it, not the day before.