The real problems of installing windows during a renovation (and how to avoid them)

If you're thinking about replacing your windows as part of a renovation, there's something you should know before signing anything: the biggest problems that arise during window installation aren't technical. They're not about the profile, or the glass, or the polyurethane foam. They're about people. About trades. About the money that moves behind the scenes.

We've been installing windows in all kinds of renovations for over 25 years — from 60 m² flats to entire farmhouses on the Costa Brava — and we've seen it all. We've seen projects that flow like silk and projects that turn into battlefields. And the difference is almost never the window. The difference is how the trades are organised, who hires whom, and whether responsibilities are clear from the start.

This article isn't about technical data or product specs. It's about what really happens on site. About the conflicts nobody tells you about until you experience them. And about how you can avoid them if you know where to look.

The conflict between the window installer and the builder: the root of the problem

Picture this scene: you're renovating your flat. You have a builder or renovation company managing the project. On your own initiative, you've found a specialist window installer because you want quality windows, properly installed, with a direct guarantee from the professional. It's a smart decision. But from here, the problems begin.

The builder's plasterer arrives on site and finds that the windows are being installed by someone "from outside". Someone who isn't part of their team. Someone they don't control. And then the phrases every homeowner has heard at some point begin: *"Look what they've done here"*, *"My installer would have done it better"*, *"That's not how it's done"*.

Sound familiar? It's the most repeated script in renovations across Spain. And it's no coincidence. Behind those phrases lies a conflict of interest that has little to do with work quality and everything to do with who gets the commission.

When the builder manages the window installation through their own team, they keep a commission. It's the standard way of operating in the industry: the builder subcontracts the installer, sets a price and pockets the difference. It's legitimate. But when the homeowner hires the window installer directly, that commission disappears. And that's where the trouble starts.

The builder's hidden commission: what nobody tells you

Let's be frank, because this is something the entire industry knows but hardly anyone says out loud: when a builder offers to include the windows in their renovation quote, they're taking a commission that typically ranges between 15% and 30% of the window price.

In other words, if your windows cost €8,000, the builder may be adding between €1,200 and €2,400 to the bill just for "managing" the installation. Not for installing. Not for guaranteeing. Just for subcontracting another professional and keeping the difference.

And here's the worst part: when the commission squeezes the margin, someone has to absorb that difference. Who? Usually the subcontracted installer — who accepts a lower margin to keep the job. And how do they compensate? By cutting where they can. And where they cut most is on materials and time spent on the installation.

When you hire a specialist installer directly, that commission margin disappears. The installer charges what they charge, with no middlemen. And that means they can use top-quality materials and take the time needed for an impeccable installation. But it also means the builder loses their commission. And in many projects, that creates friction.

When the commission eats into your window quality

This is something we've seen hundreds of times and it pains us as professionals: the builder has a fixed budget with the homeowner. They've promised "PVC windows" without specifying brand, profile or glass type. The homeowner trusts them. And the builder seeks the cheapest supplier possible to maximise their margin.

The result? Windows with a low-end profile (60 mm, 3 chambers, no steel reinforcement), standard glass without functional coatings, installation with conventional polyurethane foam instead of professional self-expanding tape, and basic hardware that degrades within a few years.

The differences may not be visible on installation day. But you'll notice them the first winter: draughts, condensation on the glass, noise that shouldn't be getting through. These are the areas where cuts happen most when the commission calls the shots:

• Profiles: from a 76-82 mm profile with 5-7 chambers and galvanised steel reinforcement… to a 60 mm profile with 3 chambers and no reinforcement
• Glass: from double glazing with low-e, solar control and argon gas… to a standard 4/16/4 double without functional coatings
• Hardware: from certified hardware with micro-ventilation and anti-lever security… to generic hardware with no features
• Installation: from a perimeter seal with Passivhaus-certified self-expanding tape… to bulk polyurethane foam that degrades in 5-8 years

The builder's commission doesn't come from nowhere. It comes from your window. It comes from the quality of materials that you'll have in your home for the next 30 years. And that has a direct impact on your comfort, your energy bill and your property value. If you want to better understand how a good window is built, we recommend our article on the best PVC window manufacturers.

Paint and plaster finishing: the dispute nobody expects

There's another problem that comes up in practically every renovation where windows are replaced, and it catches many homeowners off guard: paint and plaster finishing isn't included in the window installation. And when the homeowner sees the wall with unfinished plaster around the new window, the argument begins.

Let's explain exactly how the process works so there are no misunderstandings:

What's included and what's not included in a window installation?

• Removal of the old window: included. The installer removes and takes away the old window
• New window installation + sealing: included. Positioning, levelling, mechanical fixing and perimeter sealing with self-expanding tape or foam
• Plaster coating: normally included as basic plastering. The installer — or the plasterer assisting them — coats the wall with plaster to leave a smooth finish. This is where the installation ends
• Fine filler, sanding and painting: NOT included. This is the final finish that's the painter's job. The plaster needs a final coat of fine filler, sanding and painting to look perfect. This is the painter's work, not the window installer's

The misunderstanding arises because many homeowners expect the window to be installed and the wall to look "as good as new" — painting included. And when they see the wall with plaster that hasn't been filled or painted, they think the installer has left the job half-done. That's not the case. The installer has completed their work: the window is installed, sealed and the wall is plastered. The final step of filling and painting is a separate task that has always been the painter's responsibility.

Our advice: before the work begins, put in writing what the window installation includes and what it doesn't. If you need the finish to include filler and painting, request it as an extra and budget it separately. That way there'll be no surprises or arguments when the installer finishes their work.

Define responsibilities BEFORE starting the project

90% of the conflicts we've seen on site could have been avoided with a simple conversation at the start. A conversation where it's made clear who does what, who pays for what and who is responsible for what. Here's a clear guide:

Window installer's responsibility

• Taking measurements of the opening
• Supply and installation of the new window
• Professional perimeter sealing (self-expanding tape or foam + sealant)
• Plaster coating of the surrounding wall (basic plastering)
• Removal of the old window and cleanup of generated debris

Builder / plasterer's responsibility

• Preparing the opening if it needs enlarging or reducing (masonry work)
• Coordinating trades in the general renovation
• Exterior sill finishing if additional work is required

Homeowner's responsibility

• Hiring a painter for fine filler, sanding and painting of the affected area
• Making clear in the renovation budget whether paint finishing is included or not
• Communicating to builder and installer that they need to coordinate with each other

If these responsibilities are clear from minute zero, problems reduce dramatically. There are no arguments about finishes, no "my installer would have done it better" and no surprises on the bill.

5 tips to avoid problems when installing windows during a renovation

After 25 years of seeing everything on site, these are our most practical tips to make your window replacement a positive experience:

1. Hire the window installer directly. Don't let the builder include them in their quote without specifying brand, profile, glass and installation. Request a detailed quote from a specialist installer and compare. You'll save the middleman's commission and get a direct guarantee from the professional who installs.
2. Ask for a quote that details EVERYTHING. Profile brand, model, thickness, glass type, sealing type, whether basic plastering is included, whether paint finishing is included. If something isn't in the quote, it's not included.
3. Talk to the builder before starting. Explain that you'll be managing the windows directly with a specialist installer. That you need the plasterer to coordinate with the installer to have the openings prepared. The sooner they know, the less conflict there'll be.
4. Schedule the installation at the right point in the project. Ideally, install the windows after the masonry and before the painting. That way the painter fills and paints everything together, including the area around the windows. This avoids touch-ups and arguments.
5. Demand a written guarantee. Any professional installer should give you a written guarantee covering the installation, sealing and materials. If the builder subcontracts the windows, ask who provides the guarantee: the builder or the installer? The answer will often surprise you.

And one last tip worth its weight in gold: be wary of a builder who insists that the windows must be fitted by "their team". Ask yourself why. The answer, in most cases, begins and ends with the commission.

The real savings of hiring your window installer directly

Let's talk numbers, because ultimately this comes down to money. If you contract the windows through the builder, you're paying a surcharge that typically ranges from 15% to 30%. For a complete window replacement in an average home (6-8 windows), we're talking about a difference of between €1,000 and €3,000.

But the savings aren't just financial. When you hire the specialist installer directly, you have a direct guarantee from the person doing the work. If there's a problem with the sealing, hardware or a sash opening, you know exactly who to call. No middlemen, no excuses like "ask my subcontractor about that".

Furthermore, an installer who works directly for you — not for a builder squeezing their margin — can dedicate the necessary time to each window. They can use top-quality materials, install with certified self-expanding tape, adjust hardware precisely and leave you with a window that will work perfectly for decades.

And most importantly: a specialist installer has a reputation to protect. Their name is on the line with every project. That drives them to care about every detail. An anonymous subcontractor working on minimum margins for a builder… doesn't have the same motivation.

Conclusion: the best window is one installed without conflicts

The real problems in window installation during renovation aren't technical — they're human. They're conflicts of interest between trades, commissions nobody makes transparent and responsibilities nobody defines. And the one who ends up paying for those inefficiencies is you, the homeowner.

Our recommendation, after more than 25 years in the industry: hire a specialist installer directly, put responsibilities in writing and schedule the installation at the right point in the project. With those three things, you'll avoid 90% of the problems.

And if you're looking for a professional to install your windows with a guarantee, top-quality materials and no middlemen, request a no-obligation quote from us. We'll explain exactly what our installation includes, what it doesn't, and what your painter will need to do afterwards. No surprises.

Frequently asked questions about window installation in renovations

Is paint finishing included in the window installation?

No, typically not. Window installation includes fitting, perimeter sealing and plaster coating of the surrounding wall. But the final coat of fine filler, sanding and painting is the painter's job and is the homeowner's responsibility. It's important to make this clear before starting the project to avoid misunderstandings.

Why does the builder want to supply the windows themselves?

In most cases, because they keep a middleman's commission that typically ranges between 15% and 30% of the window price. When you hire the installer directly, that commission disappears. The builder loses income and that creates friction, which sometimes manifests as criticism of the external installer's work.

Do I save money by hiring the window installer directly?

Yes. By eliminating the middleman's commission, the saving for a complete window replacement in an average home can be between €1,000 and €3,000. Additionally, working directly with the installer gives you a direct professional guarantee, without middlemen, and generally better quality materials and installation.

What are the window installer's responsibilities versus the builder's?

The installer is responsible for removing the old window, installing the new one, professional sealing and plaster coating the wall (basic plastering). The builder handles preparing the opening if it needs masonry modifications (enlarging, reducing, reinforcing). And the homeowner is responsible for hiring a painter for the final filler and paint finish.

When is the best time to install windows during a renovation?

Ideally, install windows after the main masonry work and before the painting phase. This way, the painter can fill and paint the area around the windows along with the rest of the project, avoiding later touch-ups and disputes about who handles the finishing.

How can I avoid conflicts between the builder and the window installer?

The key is prior communication. Before the work begins, talk to the builder and explain that you'll manage the windows directly with a specialist installer. Ask the plasterer to coordinate with the installer to have the openings prepared. And put each trade's responsibilities in writing: what the installer does, what the builder does and what the painter handles. With everything defined from the start, conflicts reduce dramatically.