As Nairobi becomes more densely built up, buildings, car parks, paved compounds, and road infrastructure are steadily replacing open spaces, gardens, and vegetated areas. While this pattern of urban development supports housing and economic activity, it also transfers the responsibility of managing stormwater to public drainage systems that were never designed to accommodate such a rapid increase in surface runoff. The result is increasingly frequent local flooding, overloaded drainage networks, declining water quality, and rising infrastructure costs.

Traditionally, planning systems have responded through development standards such as plot ratios, site coverage limits, and open space requirements. These controls remain important, but they tell us little about how a site actually performs environmentally. Two developments can comply with the same zoning regulations while generating very different volumes of runoff depending on their vegetation cover, permeability, and surface treatment. As climate change intensifies flood risks, planners need tools that measure environmental performance rather than simply regulating building form.

One such tool is the Biotope Area Factor (BAF), developed through Berlin’s Landscape Plan in the late 1980s. Unlike conventional development control instruments that focus on density and coverage, BAF evaluates how effectively urban surfaces absorb, retain, and manage rainfall.

This article explores how the concept can be adapted to Nairobi’s 85 wards using satellite imagery and spatial analysis. The objective is not to replicate Berlin’s parcel-level methodology, but to test whether ecological performance can be measured consistently at city scale and used to support development control, flood-risk management, and future stormwater financing mechanisms.

Because parcel-level surface data is not available across Nairobi, the analysis relies on remotely sensed classification of permeable and non-permeable surfaces. The resulting BAF values should therefore be interpreted as a city-wide screening tool rather than parcel-level calculations.

Using Sentinel-2 imagery, GIS, and Python, the analysis identifies where ecological capacity remains strong, where it has been eroded by urbanisation, and what this means for stormwater management across the city.

The full methodology, code, and reproducible workflow are available on GitHub:

GitHub Repository: Siam3h/beyond-zoning

Ward-Level Ecological Performance

Citywide, BAF scores are uneven. The average score is 0.43, meaning that less than half of Nairobi’s land surface is functioning effectively in terms of infiltration and stormwater retention. The median is even lower at 0.37, indicating that a small number of highly vegetated wards lift the county average.

Scores range from 0.05 in the most sealed wards to 0.98 in the greenest parts of the city. Dense, paved neighbourhoods transfer rainfall directly into drainage systems, while wards with large areas of vegetation and open space absorb and slow runoff.

A quarter of Nairobi’s wards record BAF values below 0.20, while the top quartile exceeds 0.63. This ecological capacity is highly concentrated rather than evenly distributed. Wards such as Mugumo-ini, Karen, Karura, and Kitisuru record some of the highest scores in the county, reflecting large plot sizes, substantial tree cover, and relatively low levels of surface sealing.

From Ecological Performance to Stormwater Tariffs

To explore how ecological conditions could inform development control, the ward-level BAF results were translated into four conceptual stormwater tariff bands.

Under a conceptual stormwater financing framework, areas that generate more runoff would face greater obligations for mitigation, while areas already providing strong environmental functions would face lower obligations or be exempt altogether.

The largest group falls within the High Tariff Band, which contains 32 wards (37.6%). These wards have BAF values below 0.30, indicating limited infiltration capacity and a high degree of surface sealing. More than one-third of Nairobi’s wards therefore have relatively little remaining capacity to manage rainfall naturally.

A further 23 wards (27.1%) fall within the Medium Tariff Band. Together, the Medium and High bands account for nearly two-thirds of all wards in the county.

At the opposite end of the spectrum, 17 wards (20.0%) fall within the Exempt Band. These include Mugumo-ini, Karen, Karura, Kitisuru, and Mutu-ini, where vegetation and open space continue to provide strong stormwater regulation functions. Another 13 wards (15.3%) fall within the Low Tariff Band.

Population Exposure to Ecological Deficit

The runoff exposure analysis combines ecological deficit (1 − BAF) with projected population to identify wards where large numbers of residents are exposed to highly sealed urban environments.

The highest exposure scores are concentrated in eastern Nairobi and parts of Kibra and Mathare. Lindi, Kayole South, Kayole Central, Umoja II, and Upper Savanna emerge as the most exposed wards. While several wards record similarly high ecological deficits, population size plays a major role in determining overall exposure.

A clear cluster appears across the Kayole–Embakasi corridor, where multiple wards rank among the county’s most exposed locations. This suggests that stormwater risk is shaped not only by individual developments but by the cumulative effects of urban form and widespread surface sealing.

The correlation analysis reinforces this finding. Runoff exposure is more strongly associated with ecological deficit (r = 0.805) than population alone (r = 0.677), indicating that the loss of permeable and vegetated surfaces is the primary driver of exposure, while population density amplifies its impacts.

Development Control Implications

The results show that a one-size-fits-all approach to stormwater management is unlikely to work across Nairobi. Ecological conditions vary considerably between wards, meaning development controls should be calibrated to local runoff risks and infiltration capacity.

Thirty-two of Nairobi’s 85 wards (37.6%) fall within the High Tariff Band, indicating critically low capacity to absorb rainfall. In these areas, flood risk is driven as much by land-use patterns and surface sealing as it is by drainage infrastructure.

Priority interventions for High Tariff wards could include:

• Minimum permeability requirements for new developments.
• Reduced maximum site coverage ratios.
• Stormwater impact fees linked to ecological deficit.
• Incentives for green roofs and rainwater harvesting.
• Permeable paving, bioswales, rain gardens, and urban tree planting.
• Targeted investment in local drainage and green infrastructure projects.

The runoff exposure analysis further suggests that investment should be prioritised in wards where ecological deficits coincide with large populations, particularly within parts of Kayole, Embakasi, Kibra, and Mathare.

Implementation will not be straightforward. Funding constraints, technical capacity limitations, and stakeholder resistance may slow adoption. Any future stormwater financing mechanism would therefore need to be accompanied by clear regulatory guidance, capacity building, and transparent reinvestment of revenues into local flood mitigation and greening projects.

At this stage, the BAF should be viewed as a screening and decision-support tool rather than a regulatory standard. Its value lies in helping planners identify where ecological capacity has been most eroded and where interventions are likely to deliver the greatest benefit.

Limitations and Methodological Notes

Several limitations should be acknowledged:

• The analysis uses a simplified BAF proxy rather than Berlin’s full parcel-level methodology.
• Ecological performance is derived from a binary classification of vegetated and non-vegetated surfaces using Sentinel-2 imagery.
• Different surface types are not assigned separate ecological weights.
• Results are aggregated at ward level and do not capture parcel-scale variation.
• Population figures are projected estimates rather than observed counts.
• The analysis has not been validated against detailed land-cover surveys or historical flood records.

The findings should therefore be interpreted as a city-wide screening and planning tool rather than a regulatory standard.

Future work could incorporate higher-resolution imagery, cadastral datasets, field validation, and locally calibrated ecological weighting factors.

Nairobi City County’s newly gazetted Development Control Policy 2026 reflects this shift through the introduction of two key instruments: Development Impact Fees (DIF) and Transferable Development Rights (TDRs), commonly known as Air Rights. Development Impact Fees require developers to contribute towards infrastructure needed to support additional density, while Transferable Development Rights allow landowners in protected or low-density areas to sell unused development potential to developers in designated growth zones.

The significance of Nairobi’s Development Control Policy 2026 lies in how Development Impact Fees (DIFs) and Transferable Development Rights (TDRs) are designed to work together to finance infrastructure, influence housing markets, and direct urban growth. This article examines the county’s proposed framework and draws lessons from New York, São Paulo, and Mumbai to assess the opportunities and challenges associated with implementing both instruments within a single development control regime.

The Architecture of Nairobi City County’s Development Impact Fees and Transferable Development Rights

Before assessing the potential impacts of the policy, it is important to understand how the two instruments are designed to operate. While both seek to influence urban growth, they serve different functions within the development control framework. Development Impact Fees are intended to capture a portion of the infrastructure costs generated by new development, while Transferable Development Rights create a market mechanism through which development potential can be transferred from protected areas to designated growth zones.

a) Development Impact Fees: Financing Infrastructure Through Growth

The Development Impact Fee is conceived as a mandatory, one-time contribution paid by developers to offset the infrastructure demands generated by a new development. In contrast to traditional models that rely heavily on public budgets or donor funding, the DIF seeks to establish a self-financing urban infrastructure framework in which growth directly contributes to the cost of supporting growth.

Under the policy, payment of the fee is triggered at the Building Permit stage and is a prerequisite for both development approval and eventual occupancy certification. The fee will be calculated through the county’s e-permit platform using a formula that incorporates gross floor area, land-use category, and location-specific infrastructure pressure.

DF = (Gross Floor Area) × (Use Class Factor) × (Location Factor)

As a result, high-intensity commercial developments in congested areas are likely to attract higher charges, while strategic regeneration areas may benefit from lower rates intended to stimulate investment.

Revenue collected from the fee will be ring-fenced within the Nairobi Urban Infrastructure Reinvestment Fund (NUIRF), a special revenue fund established to finance roads, drainage systems, sewerage networks, water infrastructure, and public amenities. By directly linking development approvals to infrastructure financing, the county hopes to address a long-standing challenge in Nairobi’s urban development pattern: rapidly increasing densities without corresponding investment in supporting infrastructure.

b) Transferable Development Rights: Creating a Market for Density

Complementing the DIF framework is the introduction of Transferable Development Rights, commonly referred to as Air Rights. Rather than treating development potential as a fixed entitlement attached to a parcel of land, the policy allows development rights to be separated, traded, and transferred between designated locations.

Under the proposed framework, low-density residential neighbourhoods, heritage areas, environmental protection zones, green corridors, and riparian reserves function as sending zones. Property owners within these areas can monetise unused development potential by transferring it to developers operating in designated receiving zones.

These receiving zones include major growth centres such as the CBD, Upper Hill, Westlands, and Transit-Oriented Development corridors located within 800 metres of major public transport stations. Developers purchasing additional rights can exceed standard density controls by acquiring additional Floor Area Ratio (FAR) or building height allowances beyond baseline planning limits.

In practical terms, the framework transforms density into a tradable economic asset. Landowners in protected areas are compensated without requiring direct public expenditure, while developers gain access to additional development capacity in strategically important growth locations.

To support the system, the county intends to establish a formal valuation and registration framework through a future Physical and Land Use Planning By-Law, alongside a digitised register of development permissions managed through the Nairobi Planning and Development Management System. The long-term success of the market will depend on transparent valuation methodologies, efficient administration, and sustained demand for higher-density development within receiving zones.

Case Studies

The introduction of Development Impact Fees (DIFs) and Transferable Development Rights (TDRs) places Nairobi within a broader tradition of cities using market-based planning instruments to influence urban growth. While the two mechanisms pursue different objectives, experiences from New York and São Paulo demonstrate how development rights can be leveraged to achieve both spatial and fiscal planning goals.

a) New York: Preserving Urban Assets While Directing Growth

New York’s Transferable Development Rights (TDR) system emerged as a response to the challenge of accommodating growth while protecting landmarks and other valued urban assets. Under the framework, property owners can transfer unused development potential from protected sites to designated receiving areas, allowing density to be concentrated where it is most appropriate.

The most prominent example is Grand Central Terminal, where unused development rights were transferred to neighbouring properties, enabling significant densification in Midtown Manhattan without compromising a historic landmark. Over time, the system evolved into a broader growth management tool, directing development towards areas with greater capacity while preserving heritage assets and established urban character elsewhere.

For Nairobi, the relevance of the New York model lies in its potential to balance conservation and development. By allowing development rights to be transferred from low-density residential areas, heritage precincts, environmental protection zones, and green corridors to designated growth centres, the county can accommodate additional density without relying solely on restrictive zoning controls.

b) São Paulo: Linking Density to Infrastructure Investment

While New York demonstrates how development rights can shape urban form, São Paulo illustrates how they can be used to finance urban infrastructure. Through its CEPAC programme, the municipality creates and sells additional development rights within designated urban redevelopment areas, capturing part of the value generated by planning decisions.

Revenue generated through the programme has financed major infrastructure investments, including transport improvements, public spaces, drainage systems, and other urban services. The model reflects a fundamental principle of contemporary planning: areas benefiting from increased development potential should contribute towards the infrastructure required to support that growth.

However, São Paulo’s experience also highlights important implementation challenges. The effectiveness of value-capture mechanisms depends heavily on market demand, transparent governance, and institutional capacity. When property markets weaken or development rights are poorly priced, revenue generation can decline significantly.

c) Mumbai: Leveraging Development Rights for Urban Renewal

Mumbai provides a distinct example of how transferable development rights can be used to advance broader urban development objectives beyond preservation and infrastructure financing. Since the 1990s, the city has utilised TDRs to support slum rehabilitation, public infrastructure projects, and heritage conservation.

Under Mumbai’s Slum Rehabilitation Scheme, developers who construct replacement housing for informal settlement residents are granted additional Floor Space Index (FSI) in the form of transferable development rights. These rights can be used on other sites or sold to developers seeking additional development potential elsewhere in the city. Similarly, owners of heritage buildings restricted from redevelopment can generate and sell development rights, creating a financial incentive for conservation without direct public expenditure.

The Mumbai experience demonstrates how development rights can function as a planning currency, enabling cities to deliver public benefits through market mechanisms. However, it also highlights a key challenge. In several receiving zones, the concentration of additional density has outpaced infrastructure investment, contributing to congestion, pressure on utilities, and declining levels of service.

What This Means for Nairobi

Together, these cases show that development rights can perform three distinct functions: financing infrastructure, preserving urban assets, and delivering social objectives.

a) Can Growth Pay for Growth?

Kilimani, Kileleshwa, and parts of Westlands have densified rapidly over the past decade with little corresponding investment in roads, drainage, sewerage, or water infrastructure. Development Impact Fees are the county’s attempt to internalise these costs rather than leave them to public budgets. São Paulo shows the scale this can reach when managed transparently. It also shows the risk: Nairobi’s property market is comparatively thin and less liquid, making revenue reliability a concern rather than a footnote.

b) Will Density Costs Land on Residents?

Developers pursuing additional density now face a stacked cost structure: land, construction, financing, the DIF, and the price of purchased development rights. Whether this constrains or merely reprices development depends on whether the value unlocked by extra density exceeds these combined costs. Where it doesn’t, costs typically pass through to buyers and tenants - a concern in Kilimani, Westlands, and Upper Hill, where affordability is already contested. Mumbai’s experience suggests the mechanism can be steered toward affordability rather than against it, but only through deliberate design: inclusionary offsets, rebates, and targeted incentives, not by default.

c) Will This Reshape Nairobi’s Urban Form?

Nairobi’s growth has historically been market-led, with planning control responding after it. Sending and receiving zones invert that logic, letting the county actively steer where density concentrates and where it is constrained - echoing New York’s approach to protecting heritage assets while channelling growth elsewhere. Done well, this could accelerate a more polycentric Nairobi built around transit-oriented nodes.

The Missing Variable

What New York and São Paulo share, and what Nairobi cannot yet assume, is functioning valuation infrastructure: reliable comparables, transparent registries, and enforcement capacity. A TDR market is only as credible as its pricing mechanism; without one, trading density becomes negotiation rather than market.

Mumbai adds a second, Nairobi-specific caution: a substantial share of the city’s urban fabric involves informal or contested land tenure, which the current framework does not address. TDRs presume a clear, transferable title - a presumption that does not hold uniformly across Nairobi.

Policy Recommendations

a) Establish a Transparent TDR Exchange Platform

A publicly accessible digital registry and trading system would improve price discovery, reduce transaction costs, and build the liquidity a functioning market requires.

b) Ring-Fence and Publicly Report DIF Expenditure

The framework’s legitimacy rests on trust. Annual public reporting - revenue collected, allocation, and infrastructure delivered - should be mandatory, so DIFs are seen as financing infrastructure rather than functioning as an additional tax.

c) Sequence Density Approvals to Infrastructure Capacity

Directly answering Mumbai’s caution: receiving zones should not be approved for additional density faster than supporting infrastructure can be delivered. Periodic capacity reviews should govern how much additional FAR or height a zone can absorb per cycle.

d) Build Independent Valuation Capacity Before Scaling

Given Nairobi’s thin property data and limited precedent for pricing development rights, the county should commission an independent valuation methodology and pilot the market in one or two receiving zones before expanding city-wide.

e) Clarify Treatment of Informal and Contested Tenure

Before finalising sending-zone designations, the county should explicitly address how the framework applies where land tenure is informal or unresolved, so TDR eligibility does not inadvertently exclude residents without formal title.

f) Monitor Housing Market Impacts

Because the combined effect of DIFs and TDRs on cost, supply, and affordability is uncertain, an ongoing monitoring framework should track outcomes and feed back into fee and pricing adjustments over time.

Conclusion

Nairobi’s Development Control Policy 2026 does more than introduce two new instruments - it reframes development rights as economic assets capable of financing infrastructure, protecting urban character, and directing growth, rather than treating density as a fixed entitlement attached to land.

International experience shows this can work, but only under conditions Nairobi has not yet established: transparent governance, liquid property markets, and institutions capable of valuing and enforcing what is being traded. São Paulo shows what disciplined value-capture can fund; New York shows how redistribution can protect what a city values while still growing; Mumbai shows both the promise of steering density toward social ends and the cost of letting it outrun infrastructure.

The real test is not whether DIFs and TDRs are gazetted correctly, but whether Nairobi builds the valuation infrastructure, enforcement capacity, and tenure clarity that make a market in density credible rather than cosmetic.