Window requirements for historic buildings in Spain: how to modernize without breaking the law

Do you live in a beautiful building in central Barcelona and want to install double-glazed windows? Have you bought an apartment in a 19th-century building in Madrid and discovered that the windows are drafty? We explore the specifics of working with historic buildings: what is allowed, what is not, how to obtain permission, and how much it will cost. We cover real-life cases and workarounds.

Historic Building Classification – What Does Your Level of Protection Mean?

Not all old buildings are historic in the eyes of the law. Protection only extends to cataloged properties, and the level of protection determines what can be done with the windows.

BIC (Bien de Interés Cultural) – the highest national level of protection. These include palaces, cathedrals, and unique buildings. Any changes require permission from the Ministry of Culture or the autonomous government. The approval process takes at least 6-12 months. Examples: Casa Batlló in Barcelona, ​​Palacio Real in Madrid.

BRL (Local Relevance Index) or BCIL (Local Interest Catalogue Index) — regional protection. Decisions are made by the autonomous community. These are significant local buildings: old casinos, theaters, mansions. Approval takes 3-6 months.

Municipal Catalogue — municipal protection, the most common category. Most buildings in historic centers fall into this group. Protection levels:

• Nivel 1 (Integral) — everything is protected: façade, structure, and interiors. Windows can only be restored.
• Nivel 2 (Structural) — façade and structure are protected. Windows can be replaced with identical ones.
• Nivel 3 (Ambiental) — the overall appearance is protected. Windows can be replaced while maintaining the style.
• Nivel 4 (Parcial) — individual elements are protected. If windows aren’t listed, they can be changed.

Conjunto Histórico — a historic zone. Even if a specific building isn’t listed but is located in a historic zone, restrictions apply. Examples: the entire old town of Toledo, or the Barrio Gótico in Barcelona.

You can find out your security level:

• On the municipality’s website in the Urbanismo/Patrimonio section
• In Catastro (but it’s not always up-to-date)
• By request to the Department of Patrimonio
• Ask a technical architect (the most reliable method)

What you can and cannot do with windows

Integral Level — restoration only:

• Restoring the original joinery
• Replacing rotted elements with identical ones
• Reinforcing the structure from the inside (not visible from the outside)
• Adding a second glazing system from the inside
• Upgrading seals and fittings

Strictly prohibited:

• Changing the material (wood to PVC)
• Changing the pattern of the sashes
• Changing the color without approval
• Installing roller shutters
• Changing the opening type

Estructural Level — replacing with a copy:

• New windows that exactly replicate the original
• Same material or visually identical
• Same sash pattern and proportions
• Modern Hardware, if not visible from the outside
• Double-glazed windows instead of single-glazed windows (if they don’t change the appearance)

Ambiental Level - Maintaining Style:

• The material may differ, if it looks similar
• PVC with a wood-effect laminate is often allowed
• Simplified window frame patterns (if they don’t distort the style)
• Color within the building or neighborhood palette
• Internal blinds instead of shutters

Technical Solutions for Historic Windows

Restoration with Improvement is the optimal approach for the Integral level. We preserve the historic frame but improve its performance.

What specialized companies do:

• Remove old layers of paint using gentle methods
• Strengthen the wood with special resins (50-100 euros/m²)
• Replace rotted parts by milling the profile
• Install modern seals in milled grooves
• Add hidden hardware for a hermetic seal
• Paint using traditional techniques (minimum 4 coats)

A complete restoration costs 600-1200 euros/m². Expensive, but this is the only legal option for buildings with maximum security.

Double ventana – a second glazing system on the inside. The original window remains, and a modern one is installed behind it.

Advantages:

• No change in appearance
• Excellent sound insulation (40-45 dB)
• Good thermal insulation (U-value 1.5-2.0)
• Relatively easy approval

Disadvantages:

• Reduces light opening
• Difficulty in ventilation
• The window sill becomes very wide
• Additional cost: 400-600 euros/m²

Vacuum insulated glass units are a high-tech solution for thin frames. Two panes of glass with a vacuum between them, total thickness 6-8 mm (similar to single pane glass).

• U-value up to 0.7 W/m²K with a thickness of 6 mm
• Can be installed in original wooden frames
• Visually indistinguishable from regular glass
• Cost: €300-500/m² for glass only
• Manufacturers: Pilkington Spacia, AGC Fineo

Thin double-glazed windows with krypton are a compromise solution. The 3-6-3 formula with krypton provides a U-value of 2.0 with a total thickness of 12 mm.

• Fits into old frames with minimal modification
• 3 times cheaper than vacuum-insulated ones
• 40% better insulation than single-glazed windows
• Suitable for protection levels 2-3

Materials — a battle of tradition and technology

Wood is the gold standard for historic buildings. Heritage commissions typically require wood for protection levels 1-2.

Modern wooden windows for historic buildings:

• Glued laminated timber from old-growth timber
• Profiles that replicate historical ones according to drawings
• Traditional joining methods (mortises, not screws)
• Natural linseed oil and linseed oil-based paints
• Cost: €800-1,500/m²

Wood-look aluminum is sometimes permitted for protection level 3. Modern sublimation technologies create a texture indistinguishable from wood from a distance of 2-3 meters.

• Profiles replicate the cross-section of wooden frames
• Decorative overlays imitate complex profiles
• Maintenance-free, unlike wood
• Cost: €500-800/m²

PVC — Generally prohibited for levels 1-2, but for level 3, some cities allow PVC with high-quality wood-effect lamination.

Requirements for PVC in historic buildings:

• Renolit lamination or equivalent with a 15-year warranty
• Profile no wider than 70 mm (narrow, like wood)
• Color coordinated with Patrimonio (usually a RAL from the historical palette)
• Imitation of historical window frames with overlays
• Cost: €400-600/m² for special profiles

Steel — Industrial-era buildings (late 19th - early 20th centuries) often require steel frames, as this was the original material.

• Thermal break required for residential buildings
• Anti-corrosion treatment with hot-dip galvanizing
• Painting with black or dark green forge paint
• Cost: €600-900/m²

The approval process is a bureaucratic quest

Step 1: Preliminary consultation (1-2 weeks) Meeting with a Patrimonio technician to discuss possible options. Free, but schedule an appointment 2-3 weeks in advance. Bring photos of windows, the apartment’s technical data sheet, and your solution ideas.

Step 2: Restoration/replacement project (2-4 weeks) Prepared by an architect specializing in heritage. Includes:

• Historical analysis of the building
• Photographic documentation of all windows
• Drawings of existing windows (measurements)
• Restoration/replacement proposal
• Justification for the intervention
• Details of new windows at a scale of 1 Project cost: €1,500-€5,000

Step 3: Submission to the Patrimonial Commission (1 week) The project is submitted through the municipality or directly to the autonomous government. Review fee: €100-€300.

Step 4: Review by the Commission (2-6 months) The commission typically meets once a month. They may request:

• Additional information
• Changes to the project
• Material samples
• A trial restoration of one window

Step 5: Obtaining a permit and work license (2-4 weeks) After approval by the Patrimonial Commission, a regular obras license must be obtained. With the commission’s approval, this is a formality.

Step 6: Work Supervision (time depends on scope) An architect or construction specialist may come to verify compliance with the design. Any deviations will result in work being stopped and new approval required.

Step 7: Final Acceptance (1-2 weeks) A construction specialist will check the results and issue a finalization certificate.

Real Cases and Solutions

Case 1: Apartment in Casa de los Gatos, Madrid (Nivel 2)

Problem: Wooden windows from 1890, rotted lower sections, drafty, noisy street.

Solution: Restoration of the lower sections of the frames with replacement wood, installation of double-glazed windows with laminated glass on the inside.

Coordination: 3 months Cost: €12,000 for 6 windows Result: Noise reduction by 35 dB, 40% savings on heating

Case 2: Apartment in Eixample, Barcelona (Nivel 3)

Problem: The original modernist windows have been lost; aluminum ones from the 1970s are installed.

Solution: New wooden windows based on archival drawings with a 4-8-4-8-4 double-glazed unit.

Coordination: 4 months (required searching for historical photos) Cost: €18,000 for 8 windows Result: Increase in apartment value by €30,000

Case 3: Penthouse in a 1920s building, Valencia (Conjunto Histórico)

Problem: The metal windows are rusty and impossible to close.

Solution: New steel windows with thermal breaks that accurately replicate the original design.

Coordination: 2 months (Valencia is more flexible) Cost: €9,000 for 5 windows Result: Modern comfort with a historical look

Subsidies for Historic Buildings

Special financing programs are available for work on historic buildings.

1% Cultural is a government program that funds up to 75% of the cost of restoration in BIC. Competition is high and the competition is annual.

Autonomous programs:

• Catalonia: Up to 50% for buildings built before 1940
• Madrid: Up to 35% for listed buildings
• Andalusia: Up to 40% for buildings in Conjuntos Históricos
• Valencia: Up to 45% plus an interest-free loan

Municipal programs:

• Barcelona: Up to €8,000 per apartment in Ciutat Vella
• Madrid: Up to €6,000 in APIRU (Integrated Rehabilitation Zones)
• Seville: Up to 50% for facades in the historic center

Tax deductions:

• 15% in IRPF for work in BIC
• 20% for energy efficiency improvements
• Up to 60% for rental housing

Subsidy requirements:

• Design by an architect with heritage experience
• Prior approval from the Patrimonio
• Three Presumptions from specialized companies
• Commitment to maintaining the result for at least 10 years

Specialized companies and craftsmen

Working with historic windows requires specialized skills. A regular window installer can cause damage.

What a company should have:

• Experience working with Patrimonio
• Carpenters with traditional carpentry skills
• Equipment for replicating historic profiles
• Knowledge of traditional painting techniques
• Insurance for work on historic buildings

Specialized companies in Spain:

• Carpintería Histórica Madrid — a leader in restoration, expensive but high-quality
• Fusteria Modernista (Barcelona) — specialists in modernism
• Ventanas de Época (Seville) — experts in Andalusian architecture
• Restauración Norte (Bilbao) — experience with industrial heritage

Cost of specialized companies:

• 50-100% more expensive than standard installers
• But includes documentation
• Guaranteed Patrimonio approval
• Support throughout the process

Common mistakes and how to avoid them

Mistake 1: Starting without approval “The neighbor changed it and nothing happened” is the worst argument. Fines of up to €60,000 plus the obligation to restore the original appearance.

Mistake 2: Cutting corners on the project A cheap project from a non-specialist = commission rejection = redesign = wasted time and money. It’s better to pay 3,000 euros to an experienced architect than 1,500 to a novice and then have to redo it.

Mistake 3: Unauthorized improvements “Let’s also make the window wider” — any deviation from the approved project = work halted and new approval required. Even a change in paint color can be a problem.

Mistake 4: Incorrect choice of materials Wood-effect PVC from Leroy Merlin will not pass the approval process for a historic building. Certified materials with a guarantee of maintaining the original appearance are required.

Mistake 5: Ignoring details Historical hardware, special glass with defects, and forged elements are all part of the building’s appearance. Replacing them with modern equivalents can be a reason for refusal.

Alternative Solutions and Life Hacks

Temporary Solutions pending approval:

• Self-adhesive seals (10-20 euros/window)
• Transparent film for winter (30 euros/window)
• Heavy curtains for heat and sound insulation
• Portable air conditioners instead of split systems

Stage-Based Restoration: If you don’t have the budget for all the windows, you can restore them in stages. The key is to create a unified project for all windows. Start with the most problematic ones (bedroom, living room).

Collaborative Projects: Agree with your neighbors on a joint project. Savings on design and labor can reach 30%. Plus, there’s a greater chance of receiving subsidies.

Mixed Solutions: Sometimes simpler solutions are allowed for courtyard-facing windows than for façade windows. You can save money by installing expensive restoration work on the façade and double-glazed windows facing the courtyard.

Prospects for Legislative Changes

Trend toward simplification to improve energy efficiency. The EU requires renovation of old housing stock, which conflicts with heritage protection.

New technologies are gradually being adopted: vacuum-sealed glass units are already permitted almost everywhere, and electrochromic glass is being discussed.

Digitalization of processes — Madrid and Barcelona are testing online project submissions to Patrimonio, reducing the deadline to one month.

Unification of criteria between autonomous regions is planned for 2027. This will simplify work in different regions.

Living in a historic building is both a privilege and a responsibility. Yes, replacing windows will be more expensive and take longer than in a regular home. But the result is the preservation of cultural heritage and a unique atmosphere, plus an increase in property value. With the right approach, it is possible to combine a historical appearance with modern comfort.