DOI: 10.5817/FAI2014-3-2 No. 3/2014
21
Modeling Financial Surplus
of the Housing Projects Developer
Tadeusz Czernik1
, Daniel Iskra2
1
University of Economics in Katowice
Faculty of Finance and Insurance, Department of Applied Mathematics
Pułaskiego 25, 40-287 Katowice, Poland
tadeusz.czernik@ue.katowice.pl
2
University of Economics in Katowice
Faculty of Finance and Insurance, Department of Applied Mathematics
Pułaskiego 25, 40-287 Katowice, Poland
daniel.iskra@ue.katowice.pl
Abstract: Recent events taking place on the housing project market provide a
strong impetus to the study of risk in housing project development. This issue is
important not only from the point of view of the developer but also his client. This
paper proposes a dynamic model of the financial surplus process. The model
takes into account the structure of the credit payments, and the random nature of
the real estate sale process (compound Poisson process: the moment of sale and
sale price), predictable and unpredictable expenses. Monte Carlo simulations were
performed in order to introduce this model. The purpose of this paper is to
present the proposed model of financial surplus that can be a starting point for
further research and analysis.
Keywords: housing projects development, financial surplus, stochastic modeling,
simulations
JEL codes: C02, C63, G33, R30
Introduction
The events of recent years in the property development market have attested to
the urgent necessity to carry out research on the risk associated with the
development of housing investments (Tworek, 2010b). The topic of risk of
construction investments is not only relevant from the point of view of developers
but also of their customers. The literature on the risk of investments related to
construction mostly covers the topic of risk arising from the contractor of the
construction project (Skorupka, 2007), (Tworek, 2010a), (Huh et al, 2012). The
problem of project scheduling with probabilistic cash inflows was discussed in e.g.
(Ozdamar et al, 1997). This paper presents a dynamic model of a financial
surplus of a property development company.
1 Methodology
For the purposes of this study the authors have assumed that the financial
surplus is the balance of the current account of the company. Therefore, negative
balance of the surplus will be interpreted as a working capital facility granted by
Financial Assets and Investing
22
the bank to the developer. In the study, the financial surplus is a random process
with the known initial value . The dynamics of the surplus is
described using the following stochastic difference equation
(1)
where
- financial surplus at time
- simulation horizon (days)
- deterministic or random cashflow of the th factor at time .
The time unit is one day, and one year comprises 360 days (no distinction
between business days and holidays was made). Additionally, the initial point of
the investment was not placed in a specified range within the calendar year.
The following cashflows were taken into account
• interest on current account - interest accrued each day (interest rate was
not applied in case of positive surplus capital)
(2)
where: - nominal interest rate of the overdraft on the current
account, - Heaviside step function
• standard aggregate costs of business activity – salaries, cost of
rental/maintenance of office space, marketing expenses etc. (tax-related
aspects were ignored)
• investment loan - tranches of the loan and installments
• costs related to the implementation of the investment
• revenue from the sale of residential premises
• other unexpected costs .
Tax-related aspects of the undertaking were ignored.
1.1 Credit interest rate
For the purposes of the simulations it was assumed that the process of spot rate
(LIBOR ON) is generated by the equation [CIR] (all stochastic differential
equations are understood in the sense of Ito (Oksendal, 2007), (Hanson, 2007))
( )N t ( ) 00N N=
1 1,k k k i
i
N N N P+ +∆ = − = ∑
kN { }0,1,...,k T∈
T
,k iP i k
( )
( )
12
12 360
1,1 1 1 1
12
k k k k
i
P I N Nθ+ +
 
  
= = + − −   
  
 
( )12
i
( )
0 0
1 0
dla x
x
dla x
θ
≤
= 
>
,2kP
,3kP
,4kP
,5kP
,6kP
No. 3/2014
23
(3)
where
- spot rate (LIBOR ON)
- Wiener process
- parameters.
The authors decided to employ CIR model because of its appealing
characteristics:
• spot rate of zero (also negative) is precluded if the is met
• CIR is an autoregressive mean-reverting model
• the formulas for the valuation of a zero-coupon bond have a simple form.
CIR model allows to analytically determine the price of a zero-coupon bond
with a face value equal to one monetary unit
(4)
where , ,
.
Since it was assumed for the purposes of the simulations that the borrowing rate
is based on the rate of LIBOR EUR 3M (the beginning value of the borrowing rate
( ) was converted to the spot rate using the following formula
(5)
where the values of the parameters , , were adopted
on an ad hoc basis.
By solving the equation the following is obtained
. (6)
( )t t t tdr r dt r dWα µ σ= − +
tr
tW
, , 0α µ σ >
2
2
1
αµ
σ
>
( ),P t T
( ) ( ) ( ),
, , tB t T r
P t T A t T e−
=
( )
( )( )
( ) ( )
( )
2
2
2
2
,T
2 1
h T t
T t h
he
A t
h h e
αµ
α σ
α
+ −
−
 
 
=  
+ + − 
 
( )
( )
( )
( ) ( )
( )
2 1
,T
2 1
T t h
T t h
e
B t
h h eα
−
−
−
=
+ + −
2 2
2h α σ= +
03WIBOR M 0r
0
90
12 90
0,360
36003 90
1 0,
12 360
B rLIBOR M
A e
−  
−  
    
+ =   
  
0.5α = 0.003µ = 0.02σ =
90
12
360
0
0
3
1
1 12
ln
90 90
0, 0,
360 360
LIBOR M
r
B A
 
  +   =  
       
    
 
Financial Assets and Investing
24
The values of LIBOR 3M are determined using the formula
. (7)
The simulations assumed that the bank updates the borrowing rate every three
months based on the current value of LIBOR 3M or based on the averaged values
LIBOR 3M from the past few days. For the purposes of the simulation a principle
was assumed based on the averaged LIBOR 3M from the last 5 quotes.
Figure 1 shows sample LIBOR3M realizations, initial value of LIBOR3M0=0.3%.
Figure 1 Sample three LIBOR3M realizations
Source: own elaboration
Some banks change the borrowing rate only where the change in the value of
LIBOR 3M (or its average) is greater than the previously set value (the formula
for an update based on LIBOR 3M for a given day)
(8)
where
- borrowing rate
, - minimum value of the increase/decrease of LIBOR 3M resulting in a
change of the borrowing rate. In general .
The following figures show sample implementations of the borrowing rate
(excluding margin), without and with changes' thresholds. For the purposes of the
0
360
360 90
0,12 90
12 90 36090
3 12 0, 1
360
B r
tLIBOR M A e
−  
 
 
 
  = −   
 
0 360 720 1080 1440 1800
0,1%
0,2%
0,3%
0,4%
0,5%
0,6%
0,7%
time [days]
( ) ( )
( ) ( )
1 1 1
1 1
3 3
3 3
k k k k k k up
k k k k down
i i LIBOR M i LIBOR M i w
LIBOR M i i LIBOR M w
θ
θ
+ + +
+ +
= + − − − +
+ − − −
ki
upw downw
up downw w≠
No. 3/2014
25
simulations the beginning value was assumed, and
, . This asymmetric selection of the thresholds is
advantageous to the bank because the bank will raise the borrowing rate, if
LIBOR 3M exceeds the borrowing rate for the previous period by at least
0.0002. The bank will lower the borrowing rate, if LIBOR 3M is lower than the
current rate by at least 0.0004. Therefore, the increase of the rate is more
likely than its decrease.
Figure 2 Sample credit interest rate realizations without thresholds
Source: own elaboration
Figure 3 Sample credit interest rate realizations with thresholds
Source: own elaboration
03 0.3%LIBOR M =
0.0002upw = 0.0004downw =
ki
ki
0 360 720 1080 1440 1800
3,8%
3,9%
4,0%
4,1%
4,2%
4,3%
4,4%
time [days]
creditinterestrate
0 360 720 1080 1440 1800
3,8%
3,9%
4,0%
4,1%
4,2%
4,3%
4,4%
time [days]
creditinterestrate
Financial Assets and Investing
26
As shown above, after application of the threshold credit rate, the realizations
become more stable (compare circle interior).
1.2 Selling process
Three types of residential premises were considered for the purposes of the
simulations. In general, it is possible to consider any number of the premise
classes. The process of the amount of the sold premises within each of the three
classes ( ) is modeled using independent non-homogenous Poisson
processes
•
• independent increments
• if then
• if then
where is the intensity of the sale process at time .
It can be demonstrated that the probability of the sale of premises of class
during time period can be expressed as
(9)
where .
The intensity of the sales process was modeled based on the shifted gamma
distribution (shifted gamma distribution is obtained by excluding the
constant)
(10)
where
- proportionality constant, parameters, gamma
function and - start time of selling class premises.
The intensity of the sales is at its peak at the time
,i tn 1,2,3i =
,0 0in =
1t∆ ( ) ( ), , ,1i t t i t i tP n n t o tλ+∆ − = = ∆ + ∆
1t∆ ( ) ( ), , 1i t t i tP n n o t+∆ − > = ∆
, 0i tλ ≥ t
k i
( ),t t t+ ∆
( )
( ) ( ),
, ,
,
!
i
k
m t ti
i t t i t
m t t
P n n k e
k
− ∆
+∆
∆
− = =
( ) ,,
t t
i i s
t
m t t dsλ
+∆
∆ = ∫
iK
( )
( ) ( )
( ),start
1
, ,start ,start
i
i i i
a
a b t ti
i t i i i
i
b
K t t e t t
a
λ θ
− − −
= − −
Γ
0iK > , 0i ia b > ( ) 1
0
ia z
ia z e dz
+∞
− −
Γ = ∫
,startit i
No. 3/2014
27
. (11)
In the simulations authors assumed that
(12)
. (13)
On the basis of (11), (12), (13) it is possible to express by
. (14)
In the simulations it was also assumed that the most likely amount of sold
premises within the analyzed time frame is equal to the total number of built
premises
(15)
where is the incomplete gamma function.
To determine the value of it was assumed that the most likely percentage of
sold premises in class until time is
. (16)
The parameter is the solution of the equation (16), which has no analytical
solutions, therefore it was necessary to apply a numerical algorithm.
Figure 4 shows the sales’ intensity (for and
days).
,max ,start
1
for 1i
i i i
i
a
t t a
b
−
= + >
1,start 2,start 3,start startt t t t= = =
1,max 2,max 3,max maxt t t t= = =
ia ib
( )max start 1 1i ia t t b= − + >
kt
il
( ) ( )( )
( )( )
max start
max start
1,
1
i i k start
i i
i
t t b b t t
K l
t t b
γ − + −
=
Γ − +
( ) 1
0
,
y
x z
x y z e dzγ − −
= ∫
ib
i qit iq
( ) ( )( )
( ) ( )( )
max start
max start max
1,
1,
i i qi start
i
i i start
t t b b t t
q
t t b b t t
γ
γ
− + −
=
− + −
ib
.,tλ 1 2 30.4, 0.6, 0.5q q q= = =
1 2 3800, 900, 850q q qt t t= = =
Financial Assets and Investing
28
Figure 4 The intensity of the sales process
Source: own elaboration
The presented algorithm generates only the sale process of the premises. The
algorithm presented below allows inclusion of the price negotiations.
Transaction prices of a square meter for each of the classes are within the
following ranges
(17)
where
- price of a square meter in class
- multiplier for class
, - respectively the minimum and maximum base price.
The minimum base price was determined on the basis of an initial simulation so
that there is probability that the final financial surplus is greater than zero.
Price was set so that there is probability that the final surplus is greater
than the initial surplus subject to a set interest rate (i.e. deposit interest rate).
Transaction prices were generated basing on a transformed beta distribution
(18)
[ ]
[ ]
[ ]
1 1 min 1 max
2 2 min 2 max
3 3 min 3 max
,
,
,
C k C k C
C k C k C
C k C k C
∈
∈
∈
iC i
ik i
minC maxC
1p
maxC 2p
iC
( )min max mini iC k C C C Y=  + −  
The intensity of the sales process
time [days]
No. 3/2014
29
where is a random variable with a beta distribution (Gentle, 2003)
and is a beta function.
From the point of view of the above considerations on the sale process of
premises and the transaction prices, the selling process is a compounded Poisson
process.
2 Results
It was assumed for the purposes of the simulations that the time unit is one day
and one year comprises 360 days (no distinction between business days and
holidays was made). The beginning value of the surplus is €0.5 million,
the planned total cost of the investment amounts to €5 million (construction cost
of €4.5 million plus other costs: marketing, etc.).
The missing capital is borrowed. For the purposes of the simulations it was
assumed that the repayment period of the loan is five years and the time of
payment of the loan is the time of the settlement of the investment (the loan is
not repaid in advance). A loan in the amount of €4.5 million was awarded in three
tranches. The first tranche is paid at the beginning of the investment and the next
ones at the end of the following half-years (the tranches were set to amount to
€0.6, €2.6, €1.8 million). The borrowing rate consisted of a fixed 3.7% margin
plus LIBOR 3M (generated by the respective process). The simulations assumed
that the repayment of the loan is deferred for one year, and during the deferment
period the debt accumulated in accordance with the assumed borrowing rate for
the given period (monthly compounding in arrears). The first non-zero installment
is repaid after a year. Together with the update of the debt to include a new
tranche the entire loan repayment plan was updated. As noted earlier, any other
costs associated with the loan, such as the commission or insurance, were
ignored.
The estimated cost of the construction amounting to €4.5 million was spread over
two years (the expected date of completion of the construction). Some costs were
covered on a monthly basis in the amount of €100,000 and the remaining amount
was paid in 4 tranches of €525,000 at the end of each half-year. Additionally two
costs of unpredicted amounts associated with the construction were generated,
each up to 5% of €4.5 million, at randomly generated times between month 6
and 25 of the investment.
Standard aggregate costs of business activity (salaries, cost of
rental/maintenance of office space, marketing expenses etc) were assumed to
amount to €100,000 plus up to (generated randomly) 10% of €100,000.
Y
( )
( )
( )
111
1
,
ii
Y
i i
f y y y
B
βα
α β
−−
= − ( )
( ) ( )
( )
, i i
i i
i i
B
α β
α β
α β
Γ Γ
=
Γ +
( )0N =
Financial Assets and Investing
30
The process of selling the premises begins 6 months after the commencement of
the investment, the sale of the premises is identified with financial flow –
payment. Three types of residential premises were considered for the purposes of
the simulations
• 100 premises with an area of 50 m2
• 50 premises with an area of 80 m2
• 25 premises with an area of 120 m2
The maximum intensity of the sale was assumed to occur after two years from
the beginning of the investment (planned completion time), whereas it was
assumed that a period of five years (the entire term of the investment) is the
period in which the most probable number of sold premises is 100% (this does
not mean, however, that all premises will be sold at that time),
Transaction prices are generated randomly within a set range. For each class the
price range of the premises are adjusted using the appropriate multiplier (
). The initial price range is adjusted based on a measure
referred to as the probability of failure to achieve the aspiration level. The
minimum price was determined so that the probability of the event that the
generated final surplus in pre-simulations (50 000) will have a value less (or
equal to) zero and will not exceed 0.1. The maximum price was generated in a
similar manner (for a probability of 0.01), whereas the aspiration level was then
set as the value of the initial surplus value subject to the demanded rate of return
(assumed at the level of 10% p.a.) for the entire period of the investment. After
the determination of the initial price range, subsequent simulations of the
implementation of the surplus were carried out, in which the price of the premises
(expressed per m2
) was generated on the basis of the previously determined
range.
The interest rate on the current account with the surplus (the interest
compounded on a daily basis) was 15% for an overdraft, interest rate was not
applied in case of positive surplus capital (that is, for positive values of the
interest rate was assumed to be 0%).
With the above assumptions, Monte Carlo simulations were carried out by
generating 50000 realizations of daily changes in surplus capital both in the presimulations
(establishment of the initial price range per m2
) and in the final
simulations. The minimum price in the pre-simulations was obtained at €1100 per
m2
, the maximum price: €1400 per m2
.
Figure 5 presents examples of financial surplus realization. First credit tranche
€500 000 is paid at time zero, hence initial amount of money is €1000 000.
1 2 31, 1.1, 1.3k k k= = =
( )N t
No. 3/2014
31
Figure 5 Sample of financial surplus realizations
Source: own elaboration
As one can see in the picture, surplus capital decreases at the start of sales.
There are only minor differences between trajectories caused by the stochastic
nature of the credit rate (small diffusion intensity). The sale process starts to
differentiate stochastic trajectories. During the first year of the sale process,
selling intensity is small and increases.
In Figure 6 we can see the probability density functions of financial surplus. As
explained above, the dispersion of financial surplus after one year should be small
(see Figure 6). After the selling process starts, the dispersion increases.
Figure 6 PDFs of financial surplus
Source: own elaboration
0 360 720 1080 1440 1800
0
1000
2000
3000
4000
5000
6000
time [days]
amount[thousands]
-1000 0 1000 2000 3000 4000 5000 6000 7000 8000
0
0.4
0.8
1.2
1.6
x 10
-3
amount (thousands)
financial surplus after the first year
-1000 0 1000 2000 3000 4000 5000 6000 7000 8000
0
0.4
0.8
1.2
1.6
x 10
-3
amount (thousands)
financial surplus after the second year
-1000 0 1000 2000 3000 4000 5000 6000 7000 8000
0
0.4
0.8
1.2
1.6
x 10
-3
amount (thousands)
financial surplus after the fourth year
-1000 0 1000 2000 3000 4000 5000 6000 7000 8000
0
0.4
0.8
1.2
1.6
x 10
-3
amount (thousands)
financial surplus after the third year
Financial Assets and Investing
32
A histogram of the final surplus (that is, the surplus after five years of the
investment, regardless of whether all the apartments will have been sold) is
shown below. The histogram shows that the settlement of the investment may
result in losses, in extreme cases even up to €0.5 million. Therefore, it should be
considered whether it is reasonable to initiate a procedure of early withdrawal
from the investment and minimizing losses. However, in the vast majority of
cases the final surplus is positive and greater than the initial value (the average
value of the financial surplus after the fifth year of the investment amounts to
€3.4 million). The average number of premises which haven’t been sold is
approximately 8.
Figure 7 Histogram of final financial surplus
Source: own elaboration
Figure 8 shows the surplus confidence interval. One can observe that dispersion
initially increases and after approximately 40 months decreases. This is because it
is likely that by that time most of the apartments were sold, the process of loan
repayment begins to dominate.
Figure 9 shows the quantile of the final financial surplus for probability levels of
0.01, 0.03, and 0.05. The quantile determined on the basis of the empirical
distribution of the final surplus amounts to: €1,3 million, €1.7 million and €2.0
million, for the respective assumed tolerance levels. The corresponding Value at
Risk (Alexander, 2008), (Iskra, 2011) figure would be analogous.
-1000 0 1000 2000 3000 4000 5000
0
500
1000
1500
2000
2500
3000
amount (thousands)
No. 3/2014
33
Figure 8 Confidence interval
Source: own elaboration
Figure 9 Quantile of financial surplus
Source: own elaboration
2.1 Occupation time and stress testing
A stress test, in financial terminology, is an analysis or simulation designed to
determine the ability of a given financial system or financial institution to deal
with an economic crisis. In the proposed economic system we will test sensitivity
of financial surplus to changes in the intensity of sales. One can also analyze
sensitivity of apartment price decrease and the increase in credit rate. We
conduct a sensitivity analysis using relative occupation time (ROT) as a risk
measure. We propose to use ROT due to the higher chance of bankruptcy
0 360 720 1 080 1 440 1 800
0
1000
2000
3000
4000
5000
6000
7000
time [days]
amount[thousands]
0.95 symmetric confidence interval average surplus
0 360 720 1080 1440 1800
0
1000
2000
3000
4000
time [days]
amount(thousands)
0.05-quantile 0.03-quantile 0.01-quantile
Financial Assets and Investing
34
resulting from the increased time duration through which the surplus remained
below zero.
Occupation time of the process is defined as follows (Bayraktar and
Young, 2010), (Cai et al. 2010):
(19)
where:
- domain (subscript denotes dependence on time – deterministic or
stochastic),
- indicator function.
By definition, the occupation time is a random process.
Figures 10 present a sample trajectory of the process and its occupation time
process in the following domains .
Figure 10 Occupation time for sample process and
Source: own elaboration
Other quantity associated with occupation time is the relative occupation time
(Czernik, 2013):
( )t
OT A t
X
0
( ) 1 ( )S
t
t A s
OT A X ds= ∫
t
A t
( )
1
1
0t
t t
A t
t t
X A
X
X A
∈
=
∉



t
X
[0.2, )A = +∞
t
X [0.2, )A = ∞
-0,6
-0,4
-0,2
0
0,2
0,4
0,6
0 0,2 0,4 0,6 0,8 1 1,2 1,4 1,6
process X occupation time
( )t
ROT A
No. 3/2014
35
(20)
Figure 11 shows histograms of the relative occupation time. of the
surplus process is determined empirically for , where the selling
intensity were not reduced and with reduction by 10%, 30% and 50%.
In the scenario without reduction and with a probability of approximately 0.996,
the financial surplus is always positive. If we reduce the intensity by 10%
probability of the event decreases to app. 0.964. In case of reduction
by 30% and 50%, the probability mentioned above decreases to about 0.25 and
zero respectively.
Figure 11 Histogram of relative occupation time ( )
Source: own elaboration
Average ROTs obtained from simulation are 0.0001, 0.002, 0.137 and 0.572,
respectively. As can be seen, the financial surplus is very sensitive to a decrease
in selling intensity. Similar conclusions can be made by observing Figure 12.
Figure 12 shows the average values and 95% symmetric confidence intervals of
financial surplus (calculated for each day).
( ) ( ) ( )
0
1 1
1 s
t
t t A s
ROT A OT A X ds
t t
= = ∫
( )tROT A
( , 0)A = −∞
0ROT >
( , 0)A = −∞
0 0.2 0.4 0.6 0.8 1
0
0.05
0.1
0.15
0.2
0.25
0.3
relative occupation time
frequency
sales intensity reduced by 50%
0 0.2 0.4 0.6 0.8 1
0
0.05
0.1
0.15
0.2
0.25
0.3
relative occupation time
frequency
sales intensity reduced by 30%
0 0.2 0.4 0.6 0.8 1
0
0.05
0.1
0.15
0.2
0.25
0.3
relative occupation time
frequency
sales intensity without reducing
0 0.2 0.4 0.6 0.8 1
0
0.05
0.1
0.15
0.2
0.25
0.3
relative occupation time
frequency
sales intensity reduced by 10%reduction
Financial Assets and Investing
36
Figure 12 Average values and 95% symmetric confidence intervals
Source: own elaboration
In the case of reducing the intensity of sales by 50%, the final value of the
surplus will almost certainly be negative.
Conclusions
The model of the financial surplus presented in this paper is only a starting point
for further considerations. As noted above, it ignores the relationships between
the quantities, does not allow for calibration on the basis of the market data
(process of the value of the sold premises, process of borrowing rates, pricedemand
relationship), does not take into account the tax aspects of the
investment and the possibility of the initiation of bankruptcy proceedings. Neither
does it allow for the adjustment on the basis of the market data (process of the
value of the sold premises, process of borrowing rates, price-demand
relationship).
For this reason, further analyses should take into account the calibration of the
model, possible decisions of the developer in the course of the investment
(additional loans, lowering the price below the assumed minimum level to avoid
insolvency, or the possibility of declaring insolvency). Additionally, the possibility
of implementing different models of borrowing rate should be considered, as well
as different models of the distribution of transaction prices or functional
relationships of the intensity of the selling process. Measures of
0 360 720 1 080 1 440 1 800
-8000
-6000
-4000
-2000
0
2000
4000
6000
8000
sales intensity without reducing
time [days]
amount[thousands]
0 360 720 1080 1440 1800
-8000
-6000
-4000
-2000
0
2000
4000
6000
8000
sales intensity reduced by 10%
time [days]
amount[thousands]
0 360 720 1080 1440 1800
-8000
-6000
-4000
-2000
0
2000
4000
6000
8000
sales intensity reduced by 30%
time [days]
amount[thousands]
0 360 720 1080 1440 1800
-8000
-6000
-4000
-2000
0
2000
4000
6000
8000
time [days]
amount[thousands]
sales intensity reduced by 50%
reduction
No. 3/2014
37
risk/attractiveness that augment the decision making with regard to the
attractiveness of the investment can also be employed.
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