A Costa Blanca renovation starts at the outside wall

A house in Jávea and a house in Haarlem solve opposite problems. One holds heat in through a long winter; the other keeps it out through a summer that softens bitumen on the roof. Arrive with Northern European instincts and the first decision goes wrong. That decision sets the cost of the rest.

Houses on this coast from the 1970s to the 1990s are typically a reinforced concrete frame — pillars, beams, slabs — with hollow clay brick filling the gaps and the cavity left empty. The frame is the difficulty. Concrete conducts, so every pillar, every slab edge and every balcony running out through the wall is a direct line between inside and outside. You can find them by hand in February: the corner of a north-facing bedroom is colder than the wall beside it.

Insulation on the wrong side

The cheap answer is a stud wall on the inside. Plasterboard, mineral wool, no scaffolding. It fails for two reasons.

The first is that the thermal bridges survive. An internal lining stops where the floor and ceiling slabs arrive, so those edges keep conducting and the cold surface moves out of sight behind the plasterboard. Damp follows it. Surface mould you can wash off. Mould in a void you cannot see announces itself by smell, long after it started.

The second is thermal mass. What makes a Spanish house tolerable in August is not insulation but weight — masonry that takes most of the day to warm through, so the heat arrives late in the evening, when you can open the windows and throw it back out. Line the inside with plasterboard and that mass sits outside your insulation. The room heats up at the speed of a caravan.

Insulating on the outside is harder and, for these houses, right. The envelope is wrapped, the frame stays on the warm side, and the wall dries outwards. It costs more, because it is not really a wall job: reveals and sills are rebuilt, and the windows ideally move into the plane of the insulation. Cantilevered balcony slabs cannot be wrapped at all, only insulated top and soffit and accepted as a compromise. Thickening a wall can also touch on setbacks and permitted footprint, a question for the architect and the town hall while the design is still on paper.

Damp, and then salt

Damp arrives from three directions and only one is familiar. The autumn storms, the gota fría, deliver in an afternoon what a Dutch autumn spreads over weeks, and a flat roof with a lazy fall and two small outlets does not drain, it fills. Then the ground. Without a membrane under the wall, moisture wicks up and blows the plaster, and waterproof render over the top moves the tidemark rather than stopping it. Then the building's own air. A house standing empty half the year has no air movement, the north walls stay cold, and mould appears in wardrobes and behind headboards.

Salt is slower. Within sight of the sea the air works on every piece of metal, including the reinforcement inside concrete balcony noses, where corrosion expands and pushes the cover off in sheets. Galvanised fixings that would be fine inland do not last here. A4 stainless outdoors, coated or anodised aluminium, and the same discipline for hinges, gate hardware, pool plant and the screws holding the shutters.

Two building seasons

Summer is not a working season for anything wet or outdoors. From July the day shifts to the early morning and output falls with the thermometer. Materials misbehave more than people do: renders and screeds lose their water before they cure, and liquid membranes go onto a roof deck too hot to kneel on. Much of the supply chain closes for weeks in August, and not the same weeks, so one missing item stalls everything behind it. Some municipalities restrict noisy work in the peak season. Check that before the programme is drawn.

That leaves spring and autumn, with one fixed point: the roof and the drainage are finished and tested before the autumn rain, not during it. Summer is for interior work, for joinery made in a workshop, and for the decisions that have to be made anyway.

Deciding before the wall comes down

At home a renovation begins with drawings and a specification, and the trades price a defined scope. Here it often starts from a short quotation with lines like "bathroom renovation" and a number beside it. Everything the quotation does not say gets decided on site, by whoever is standing there. If you are two thousand kilometres away, that is not you.

So the specification comes first, in more detail than feels necessary at home, and before anything is demolished. Not because the trades need instructions — they generally do not — but because the decisions sit physically upstream of the work. Tile format sets the substrate build-up, the build-up sets the finished floor level, and the floor level sets the door heights and the threshold to the terrace, where water gets in. Sockets and pipe runs are chased into walls before plastering, so the kitchen layout is settled before the chaser arrives, or it is settled twice. Every Rocksure project is documented this way in a specification book; a five-page extract sits at rocksure.es/en/specification-book.

Then coordination. Spanish practice leans towards independent trades rather than one contractor carrying the rest: bricklayer, plumber, electrician, carpenter, painter, each with a separate quotation and calendar. Somebody has to own the sequence. If nobody is named, the sequence owns itself: the plasterer arrives to find nothing chased, the tiler a screed still green. A question that would take thirty seconds standing in the room takes days across email and a language barrier. More supervision does not fix that. Fewer open questions do, along with one person on site who can decide inside an agreed frame, and a fixed day each week when decisions get written down.

None of this makes renovating here harder than at home. It makes it front-loaded. The climate is patient with a bad decision, then presents it all at once, years later, usually to somebody else.