Before I present my argument, I'd like to make the following assumptions;
1. that your mortgage repayments are higher than the cost of renting a house in the city
2. that you are on a variable rate of interest for your mortgage
3. that at no time will the interest rates change in the future to negate 1 above.
If the above three factors hold, then it would make economical sense to keep the house, rent it and plough into your future mortgage repayments;
(a)the extra 5k from renting the house, and
(b)the difference between the current mortgage repayment and your rent in the city
For the additional costs associated with living in Rongai (transport, security and any other cost), you can plough these into your mortgage repayments (not much at face value but over 25 years or thereabout, they will make a significant difference in your mortgage)
The other major reason for keeping the house is, at any given time, you are a home-owner and got a tangible asset to use as security for any additional loans or financial commitments (I know we are not there yet in Kenya but we are heading there - the bank or another third party can loan you money against the value of your partial ownership of the property without undue encumberance from your mortgage provider - well, unless you are not making the repayments!)
There are also the non-monetary benefits of getting home while the sun is still shining and spending as much of the evening at home and not stuck in traffic on Langata Road!
and lastly, of course, so long as hijackings continue unabated off the east coast of Africa, the property boom is here to stay and therefore you may not be in a position to afford your house 3-5 years down the road.
Just like in F1, it's all in the telemetry