What to consider before installing them
Four things: how much electricity the house will use and at what times of day, what the roof is like (orientation, shading, structure, waterproofing), where the inverter and battery will go, and how the self-consumption paperwork will be handled. All of this is decided at design stage, before the roof is waterproofed and before first-fix electrics. That way the panels become part of the house rather than an add-on.
If you leave it until the end of the build, panels can still be installed. But the fixings will have to pierce finished waterproofing, the cable will run down the façade or through freshly plastered walls, and the inverter will end up wherever there is room rather than where it belongs.
Size it from consumption, not roof area
The size of the system is set not by how many panels fit on the roof but by what the house will consume. In a new house the list of loads is known in advance: a heat pump for heating and hot water, summer air conditioning, the pool pump and filtration, an EV charger, an induction hob. Each one changes the calculation.
Timing matters too. Panels generate during the day. A seaside house in Castelldefels, with the pool and air conditioning running on summer days, matches production well. A family that is out all day and switches everything on in the evening, without a battery, exports much of its energy to the grid at an unattractive price. A battery is not mandatory; it answers the question of when you use your electricity. It makes sense to allow at least the space and the cable route for one, even if you buy it later.
The roof: orientation, shade and waterproofing
South-facing gives the best yield. East-west produces less at midday but spreads generation across morning and evening, which suits some households better. Shade from a neighbouring building, a pine tree, a parapet or a vent stack cuts output more than you would expect, so orientation and panel layout are studied together with the house design, not afterwards. How orientation affects the whole house is covered in our article on orientation and shading.
On a flat roof, panels sit on ballasted frames or on fixings through the roof. Every fixing that passes through the waterproofing is a potential leak point, so their number and details are agreed with whoever does the waterproofing and carried out in the same sequence, not by a different crew a year later. The weight of a ballasted system is checked by the structural engineer.
Cabling, inverter and battery
The cable from the roof to the inverter is run in conduit or trunking during construction, hidden inside the house. That is tens of metres that cannot be installed neatly once the finishes are on. The inverter makes some noise and gets warm: it needs a ventilated spot, not behind a bedroom wall, with access for servicing. Usually that means the plant room, the garage or a storeroom next to the consumer unit.
The battery needs a space with moderate temperatures and safe access. If you plan to have an electric car, bring the charger onto the same board from the outset, so you do not have to run a new circuit through a finished garage later. These decisions go into the house's electrical design alongside the sockets and lighting, so the whole system sits on one drawing.
Self-consumption paperwork
A self-consumption installation needs municipal permission and registration, and if you export surplus power to the grid, a compensation agreement with your electricity retailer as well. Which type of permit your municipality requires, whether there are local reductions on property or construction taxes, and how long processing takes are confirmed with the city council or a gestor. Conditions vary from one municipality to the next, so we do not quote figures in advance.
The practical advice is simple: include the panels in the house design and the building permit application from the outset, rather than processing them separately after completion. The installation paperwork will be needed later for the energy performance certificate and if you ever sell the house.
A real example and the costly mistakes
The private house with a pool that we delivered as a full-cycle project from 2022 to 2024 has solar panels on the roof as part of the original design. Their routing and connection were coordinated with the rest of the electrics during first fix. That way the cabling stays hidden and the roof does not need opening up for the panels once it is waterproofed.
The typical costly mistakes repeat from house to house: fixings through finished waterproofing without agreed details, the inverter on a bedroom wall, no safe roof access for cleaning and maintenance, standard fixings by the sea in Castelldefels or the Garraf, where salty air eats through metal. Each is solved by one line in the design and becomes expensive if discovered a year later.