/ rental price anomaly detection · madrid

Which Madrid rentals are genuinely mispriced?

An isolation forest, built from scratch, over 587 live Fotocasa listings, combined with a comparable-based fair-price model and a hedonic regression. It flags listings whose price doesn't fit their size, location, condition and features. Not the cheapest or the dearest, the anomalous.

587

listings

21 districts

€1500

median rent

€23.1/m²

underpriced

bargains

overpriced

asking too much

structural

rare, price fair

/ the map of mispricing

Each dot is a flat: its comparable (fair) rent on the horizontal axis, its actual asking rent on the vertical. On the diagonal, price matches comparables. Below the line = underpriced; above = overpriced. Only dots the isolation forest also rates anomalous are highlighted.

underpriced overpriced structural anomaly fairly priced

/ top underpriced - the bargains

Ranked by combined mispricing score: distance from comparables × isolation-forest anomaly.

Peñagrande · Fuencarral - El Pardo€1800/mo · 134 m²

€1800/mo · 134 m² · 3BR/2BA in Peñagrande (Fuencarral - El Pardo). 549 m to Peñagrande, 6.6 km to centre. Comparable flats rent for ~€2958 (€22.1/m²); this listing is 39% below comps (-2.3σ). Anomaly score 0.48. Stands out on: lon low, has_air_conditioning low, log_surface high.

-39% vs comps anomaly 0.48 view ↗

Almendrales · Usera€1300/mo · 90 m²

€1300/mo · 90 m² · 3BR/1BA in Almendrales (Usera). 439 m to Almendrales, 3.4 km to centre. Comparable flats rent for ~€1778 (€19.8/m²); this listing is 27% below comps (-1.9σ). Anomaly score 0.53. Stands out on: has_storage_room high, has_balcony high, has_terrace high.

-27% vs comps anomaly 0.53 view ↗

Pilar · Fuencarral - El Pardo€1200/mo · 75 m²

€1200/mo · 75 m² · 2BR/1BA in Pilar (Fuencarral - El Pardo). 200 m to Peñagrande, 6.6 km to centre. Comparable flats rent for ~€1667 (€22.2/m²); this listing is 28% below comps (-2.7σ). Anomaly score 0.42. Stands out on: has_terrace high, has_equipped_kitchen low, rooms low.

-28% vs comps anomaly 0.42 view ↗

Tres Olivos - Valverde · Fuencarral - El Pardo€1075/mo · 66 m²

€1075/mo · 66 m² · 1BR/1BA in Tres Olivos - Valverde (Fuencarral - El Pardo). 450 m to Fuencarral, 8.6 km to centre. Comparable flats rent for ~€1435 (€21.7/m²); this listing is 25% below comps (-2.2σ). Anomaly score 0.45. Stands out on: is_exterior low, has_elevator low, floor_code low.

-25% vs comps anomaly 0.45 view ↗

San Fermín · Usera€1200/mo · 104 m²

€1200/mo · 104 m² · 2BR/2BA in San Fermín (Usera). 466 m to San Fermín-Orcasur, 5.7 km to centre. Comparable flats rent for ~€1896 (€18.2/m²); this listing is 37% below comps (-2.0σ). Anomaly score 0.47. Stands out on: has_wardrobes low, rooms low, has_garage high.

-37% vs comps anomaly 0.47 view ↗

Almenara -Ventilla · Tetuán€1490/mo · 83 m²

€1490/mo · 83 m² · 2BR/2BA in Almenara -Ventilla (Tetuán). 566 m to Chamartín, 6.4 km to centre. Comparable flats rent for ~€1871 (€22.5/m²); this listing is 20% below comps (-1.9σ). Anomaly score 0.48. Stands out on: has_wardrobes low, has_balcony high, bathrooms high.

-20% vs comps anomaly 0.48 view ↗

Sol · Centro€900/mo · 50 m²

€900/mo · 50 m² · 1BR/1BA in Sol (Centro). 224 m to Ópera, 0.4 km to centre. Comparable flats rent for ~€1415 (€28.3/m²); this listing is 36% below comps (-2.0σ). Anomaly score 0.45. Stands out on: has_air_conditioning low, has_elevator low, noise_index low.

-36% vs comps anomaly 0.45 view ↗

Cortes - Huertas · Centro€1249/mo · 100 m²

€1249/mo · 100 m² · 3BR/1BA in Cortes - Huertas (Centro). 288 m to Antón Martín, 0.4 km to centre. Comparable flats rent for ~€2492 (€24.9/m²); this listing is 50% below comps (-1.7σ). Anomaly score 0.48. Stands out on: has_balcony high, log_surface high, has_air_conditioning low.

-50% vs comps anomaly 0.48 view ↗

Rejas · San Blas€1100/mo · 75 m²

€1100/mo · 75 m² · 1BR/1BA in Rejas (San Blas). 1826 m to Alameda de Osuna, 11.3 km to centre. Comparable flats rent for ~€1651 (€22.0/m²); this listing is 33% below comps (-1.6σ). Anomaly score 0.50. Stands out on: has_garage high, log_surface high, floor_code high.

-33% vs comps anomaly 0.50 view ↗

Pilar · Fuencarral - El Pardo€1200/mo · 70 m²

€1200/mo · 70 m² · 2BR/1BA in Pilar (Fuencarral - El Pardo). 192 m to Peñagrande, 6.8 km to centre. Comparable flats rent for ~€1550 (€22.1/m²); this listing is 23% below comps (-1.9σ). Anomaly score 0.44. Stands out on: conservation_code high, lon low, has_equipped_kitchen low.

-23% vs comps anomaly 0.44 view ↗

Almendrales · Usera€950/mo · 63 m²

€950/mo · 63 m² · 2BR/1BA in Almendrales (Usera). 194 m to Almendrales, 3.9 km to centre. Comparable flats rent for ~€1328 (€21.1/m²); this listing is 28% below comps (-1.5σ). Anomaly score 0.47. Stands out on: floor_code low, has_terrace high, is_exterior low.

-28% vs comps anomaly 0.47 view ↗

Almendrales · Usera€1760/mo · 140 m²

€1760/mo · 140 m² · 3BR/2BA in Almendrales (Usera). 260 m to Almendrales, 3.9 km to centre. Comparable flats rent for ~€2599 (€18.6/m²); this listing is 32% below comps (-1.5σ). Anomaly score 0.47. Stands out on: has_pool high, log_surface high, has_garage low.

-32% vs comps anomaly 0.47 view ↗

/ top overpriced - asking too much

Sol · Centro€3295/mo · 60 m²

€3295/mo · 60 m² · 1BR/1BA in Sol (Centro). 159 m to Sevilla, 0.4 km to centre. Comparable flats rent for ~€1500 (€25.0/m²); this listing is 120% above comps (+7.2σ). Anomaly score 0.52. Stands out on: has_pool high, is_modern high, has_equipped_kitchen low.

+120% vs comps anomaly 0.52 view ↗

Sol · Centro€3500/mo · 70 m²

€3500/mo · 70 m² · 1BR/1BA in Sol (Centro). 187 m to Gran Vía, 0.2 km to centre. Comparable flats rent for ~€1731 (€24.7/m²); this listing is 102% above comps (+5.7σ). Anomaly score 0.59. Stands out on: conservation_code high, antiquity_code low, is_furnished low.

+102% vs comps anomaly 0.59 view ↗

Embajadores - Lavapiés · Centro€3900/mo · 80 m²

€3900/mo · 80 m² · 2BR/2BA in Embajadores - Lavapiés (Centro). 27 m to La Latina, 0.8 km to centre. Comparable flats rent for ~€1801 (€22.5/m²); this listing is 117% above comps (+5.8σ). Anomaly score 0.47. Stands out on: has_garage high, has_balcony high, metro_within_500m low.

+117% vs comps anomaly 0.47 view ↗

Casco Histórico de Barajas · Barajas€1670/mo · 38 m²

€1670/mo · 38 m² · 2BR/1BA in Casco Histórico de Barajas (Barajas). 370 m to Barajas, 12.5 km to centre. Comparable flats rent for ~€859 (€22.6/m²); this listing is 94% above comps (+4.5σ). Anomaly score 0.52. Stands out on: has_wardrobes low, is_furnished low, has_equipped_kitchen low.

+94% vs comps anomaly 0.52 view ↗

Sol · Centro€2100/mo · 45 m²

€2100/mo · 45 m² · 2BR/1BA in Sol (Centro). 226 m to Sol, 0.2 km to centre. Comparable flats rent for ~€1213 (€27.0/m²); this listing is 73% above comps (+4.7σ). Anomaly score 0.50. Stands out on: has_ensuite high, rooms high, has_elevator low.

+73% vs comps anomaly 0.50 view ↗

Sol · Centro€3950/mo · 95 m²

€3950/mo · 95 m² · 2BR/1BA in Sol (Centro). 104 m to Sol, 0.1 km to centre. Comparable flats rent for ~€2228 (€23.5/m²); this listing is 77% above comps (+3.4σ). Anomaly score 0.57. Stands out on: has_storage_room high, has_ensuite high, antiquity_code low.

+77% vs comps anomaly 0.57 view ↗

Goya · Barrio de Salamanca€5000/mo · 90 m²

€5000/mo · 90 m² · 2BR/2BA in Goya (Barrio de Salamanca). 117 m to Goya, 2.4 km to centre. Comparable flats rent for ~€2097 (€23.3/m²); this listing is 138% above comps (+4.5σ). Anomaly score 0.48. Stands out on: is_exterior low, is_modern high, is_furnished high.

+138% vs comps anomaly 0.48 view ↗

Sol · Centro€1100/mo · 20 m²

€1100/mo · 20 m² · 1BR/1BA in Sol (Centro). 243 m to Sol, 0.2 km to centre. Comparable flats rent for ~€604 (€30.2/m²); this listing is 82% above comps (+5.0σ). Anomaly score 0.46. Stands out on: log_surface low, is_exterior high, conservation_code low.

+82% vs comps anomaly 0.46 view ↗

Ciudad Jardín · Chamartín€2060/mo · 38 m²

€2060/mo · 38 m² · 1BR/1BA in Ciudad Jardín (Chamartín). 297 m to Alfonso XIII, 4.4 km to centre. Comparable flats rent for ~€1011 (€26.6/m²); this listing is 104% above comps (+4.0σ). Anomaly score 0.50. Stands out on: has_pool high, has_ensuite high, lat high.

+104% vs comps anomaly 0.50 view ↗

Embajadores - Lavapiés · Centro€3450/mo · 80 m²

€3450/mo · 80 m² · 2BR/2BA in Embajadores - Lavapiés (Centro). 10 m to La Latina, 0.8 km to centre. Comparable flats rent for ~€1801 (€22.5/m²); this listing is 92% above comps (+4.6σ). Anomaly score 0.46. Stands out on: has_garage high, has_balcony high, metro_within_500m low.

+92% vs comps anomaly 0.46 view ↗

Sol · Centro€3750/mo · 95 m²

€3750/mo · 95 m² · 0BR/2BA in Sol (Centro). 276 m to Tirso de Molina, 0.5 km to centre. Comparable flats rent for ~€2119 (€22.3/m²); this listing is 77% above comps (+3.4σ). Anomaly score 0.51. Stands out on: rooms low, bathrooms high, is_exterior low.

+77% vs comps anomaly 0.51 view ↗

Los Ángeles · Villaverde€2900/mo · 60 m²

€2900/mo · 60 m² · 2BR/1BA in Los Ángeles (Villaverde). 171 m to Villaverde Bajo-Cruce, 7.2 km to centre. Comparable flats rent for ~€1250 (€20.8/m²); this listing is 132% above comps (+4.6σ). Anomaly score 0.44. Stands out on: dist_metro_m low, is_exterior low, has_elevator high.

+132% vs comps anomaly 0.44 view ↗

/ features the market systematically misprices

Among flagged listings, which features cluster with bargains vs rip-offs. A teal bar means the feature is over-represented among underpriced flats, so the market underpays for it (hidden value). A red bar means it clusters with overpriced flats, so the market overpays for it (hype).

/ what the market pays per feature

Hedonic regression: implicit €/month a feature adds to rent, holding size, location and condition constant (R² = 0.81).

feature	€/mo	effect	t
modern	+186	+12%	+4.9
a/c	+123	+8%	+3.5
furnished	+108	+7%	+3.1
exterior	-98	-7%	-2.8
pool	+75	+5%	+1.4
garage	-59	-4%	-1.4
wardrobes	-58	-4%	-1.8
equipped kitchen	+45	+3%	+1.3
balcony	+22	+1%	+0.6

/ anomaly score distribution

Isolation-forest score for every listing. The right tail (red, top decile) is where genuine anomalies live; most listings cluster as "normal".

/ method & honesty note

Data: 587 live rental listings scraped from Fotocasa (05 Jul 2026), across all 21 Madrid districts. Idealista was intended too but returns HTTP 403 to non-browser clients (DataDome), so Fotocasa, which embeds listings as JSON, was used.
Location: haversine proximity to 243 real metro stations (Wikidata), 15 parks, 10 universities, a nightlife-noise kernel and centrality. Not raw lat/lon alone.
Isolation forest: from scratch. Random split trees, path-length + c(n) normalisation, s = 2^(-E[h]/c(ψ)), no scikit-learn.
Fair price: median price/m² of each listing's 40 nearest comparables in a price-independent characteristic space; residual z-scored robustly (MAD), gated on comparable reliability.
Caveats: asking prices, not transacted rents; comparable sets are thin in low-inventory districts; condition/antiquity are coded ordinals. Flags are leads to inspect, not verdicts.