# Build or invest — the development crossover calculator

> A browser calculator at https://vibes.obel.dev/build-or-invest/ that models what a lump sum becomes if it builds a rental block versus if it compounds in the market, and reports the year one overtakes the other. Every input is encoded in the URL query string, so a link opens with the whole scenario already filled in.

This file is the complete parameter reference for building a link to this
calculator. The tables are generated from the calculator's own field
definitions — if this file and the app disagree, the app is right and this is a
bug.

## How to build a link

1. Start from `https://vibes.obel.dev/build-or-invest/`.
2. Append `?` then `&`-joined `name=value` pairs from the tables below.
3. **Only include parameters you are changing.** Anything omitted uses its
   default, so a bare `https://vibes.obel.dev/build-or-invest/` is the default scenario.
4. A link is self-contained. Opening one resets every parameter you left out
   back to its default, ignores whatever scenario the visitor had saved, and
   does not overwrite it.
5. Values are plain decimal numbers — no thousands separators, no currency
   symbol, no `%`. Negatives are fine where the range allows.
6. **All money is Kenyan shillings (KES), always.** The `c` parameter changes
   the display currency only. It does not convert the numbers you pass and it
   does not change the model, so convert to KES yourself before building the
   link.
7. Out-of-range values are never rejected, only clamped — a link stays valid
   while quietly meaning something else. Money is clamped to 0 to 1000000000000;
   everything else to the range in its table.
8. The on-screen sliders move in fixed steps, but **URL values do not have to
   be multiples of those steps.** Pass the real figure.
9. Percentages are whole-number percents: `50` means 50%, not 0.5.
10. Output the finished URL as raw text on its own line. Do not wrap it in a
    markdown link, do not percent-encode it, and do not write `&` as `&`.
11. These parameter names are specific to this calculator. Do not reuse them on
    another vibes calculator — several short names mean different things there.

## Modes

`m` chooses which rental tax regime applies. Default `gross`. Case-sensitive, and any value other than `net` is treated as `gross`.

| value | shown as | means |
| --- | --- | --- |
| `gross` | Flat rate on rent | A flat percentage of rent collected, before any costs — Kenya's residential rental regime. Uses `flat`. |
| `net` | Tax on profit | Your marginal income tax rate applied to net rental profit after operating costs. Uses `mtx`. |

`flat` is ignored under `net`, and `mtx` is ignored under `gross`. Setting the wrong one for the mode does nothing.

## Display currency

`c` sets the currency results are *displayed* in. Default `KES`. Accepted values: `KES`, `UGX`, `USD`, `GBP`, `EUR`, `ZAR`, `NGN`, `INR`, `AED`. An unrecognised code is ignored entirely. Display only — see rule 6.

## Parameters

### What you're deploying

| param | means | default | range | notes |
| --- | --- | --- | --- | --- |
| `cap` | Money you have to deploy | 40000000 | KES | The same lump sum goes into the building or into the market. That's the whole comparison. |

### The project

| param | means | default | range | notes |
| --- | --- | --- | --- | --- |
| `land` | Land cost | 8000000 | KES | Set to 0 if you already own the plot. |
| `u` | Number of units | 12 | 1 to 200 |  |
| `cpu` | Build cost per unit | 2200000 | KES | Shell, finishes and services, per unit. |
| `fee` | Design, approvals & supervision | 8 | 0 to 25 % | Architect, engineer, county approvals, NEMA. A share of build cost. |
| `cont` | Contingency | 10 | 0 to 30 % | Overruns are the norm, not the exception. |
| `bm` | Months to build | 18 | 1 to 60 months | No rent arrives during these. This is the drag spreadsheets forget. |
| `lm` | Months to fill it | 6 | 0 to 36 months | Occupancy ramps from empty to your long-run vacancy over this stretch. |

### What it earns

| param | means | default | range | notes |
| --- | --- | --- | --- | --- |
| `r` | Rent per unit | 35000 | KES | Per month, when occupied. |
| `vac` | Long-run vacancy | 8 | 0 to 40 % | Share of the year a unit sits empty once the building has settled. |
| `mgmt` | Agent's cut | 8 | 0 to 25 % | Letting and management fees on rent collected. |
| `rg` | Rent growth | 5 | -5 to 20 % | Per year. This also drives your exit value, so it works twice. |

### Cost of running it

| param | means | default | range | notes |
| --- | --- | --- | --- | --- |
| `rt` | Land rates | 0.2 | 0 to 4 % | Per year, as a share of the site value. |
| `ins` | Insurance | 180000 | KES | Per year, for the whole building. |
| `rep` | Repairs & upkeep | 1 | 0 to 5 % | Per year, as a share of build cost. 1% is the usual rule of thumb. |
| `cc` | Common areas & security | 60000 | KES | Per month. Caretaker, water, lifts, grounds, lighting. |

### Getting out

| param | means | default | range | notes |
| --- | --- | --- | --- | --- |
| `cr` | Exit yield | 8 | 1 to 20 % | What a buyer demands. The building is worth a year's net income divided by this — the single biggest lever on your exit. |
| `sp` | Selling costs | 3 | 0 to 15 % | Agent and legal fees when you sell. |
| `h` | Years before you sell | 10 | 1 to 40 years |  |

### The alternative

| param | means | default | range | notes |
| --- | --- | --- | --- | --- |
| `inv` | Return if invested instead | 13 | 0 to 25 % | Annual, compounding. Whatever you'd actually earn leaving the money in the market. |
| `itx` | Tax on those returns | 15 | 0 to 40 % | Withholding tax. 15% on most Kenyan unit trusts. |
| `ife` | Annual management fee | 1 | 0 to 5 % | Charged on the balance, so it bites every year. |

### Tax on rent — mode `gross` only

| param | means | default | range | notes |
| --- | --- | --- | --- | --- |
| `flat` | Flat rate on gross rent | 7.5 | 0 to 20 % | Kenya's residential rental regime — a share of what you collect, before any costs. |

### Tax on profit — mode `net` only

| param | means | default | range | notes |
| --- | --- | --- | --- | --- |
| `mtx` | Your income tax rate | 30 | 0 to 60 % | Applied to net rental profit after operating costs. |

### Tax & inflation

| param | means | default | range | notes |
| --- | --- | --- | --- | --- |
| `cgt` | Capital gains tax | 15 | 0 to 40 % | On the gain over total project cost when you sell. |
| `infl` | Inflation | 6 | 0 to 20 % | Pushes up insurance, repairs and common-area costs. |

## Worked examples

An 8-unit block in Kikuyu: KES 30M to deploy, KES 6M for the plot, KES 1.9M a unit, renting at KES 28,000:
https://vibes.obel.dev/build-or-invest/?cap=30000000&land=6000000&u=8&cpu=1900000&r=28000

A 24-unit development taxed on profit, sold after 15 years at a 9% exit yield:
https://vibes.obel.dev/build-or-invest/?u=24&cap=80000000&cpu=2600000&h=15&cr=9&m=net

The same 8-unit block, but the market alternative returns 16% instead of 13%:
https://vibes.obel.dev/build-or-invest/?cap=30000000&land=6000000&u=8&cpu=1900000&r=28000&inv=16

## What the calculator does with these

The same lump sum takes both paths. On the build path it buys land, pays for
construction over `bm` months earning nothing, then ramps from empty to `vac`
occupancy over `lm` months; whatever hasn't been drawn yet keeps compounding while
it waits. The building's exit value is a stabilised year's net income divided by
`cr`, less selling costs and capital gains tax. On the market path the same money
compounds at `inv`, net of `itx` and `ife`.

The page reports which path wins, the crossover year, the project's IRR and yield
on cost, whether the lump sum actually covers the project, and which single input
would flip the verdict.

`cr` is the biggest lever on the build side and `inv` on the other — a point of
exit yield moves the answer more than most of the construction budget.

The defaults are Kenyan, but nothing is hardcoded to Kenya. Every tax and rate is
a plain input.

Not modelled: construction finance or any debt at all — this compares cash against
cash. Also absent: phased sales of individual units, ground rent, and the risk that
a project simply doesn't finish. It is a model, not advice.

## Related

- https://vibes.obel.dev/llms.txt — index of the other tools
- https://vibes.obel.dev/rent-or-buy/llms.txt — the same comparison for a single home you'd live in