Optimal timing and investment strategies are critical in markets when project demand is uncertain. When developer has more than one project located in close proximity to each other, interactive effects will have significant effects on the optimal timing decision. If positive interactive effects can be created by integrating the two projects, the values of the two projects could be collectively enhanced when they are developed jointly by the developer. Similarly, if the projects are developed by two competing developers, development strategy may be adopted such that the completion of one project may create negative externality on the neighboring project owned by another developer. This paper aims to develop a deterministic model to examine multiproject interactive effects on developer's investment and development strategies. The model will also evaluate how investment strategies change under different market situation and for different project type, either homogeneous or heterogeneous. Under the constant demand and cost functions, portfolio effects of multiple projects are examined. The portfolio effects, in this context, refer to the spill-over benefits generated by second projects when two projects located in close proximity enjoy positive externality. The spill-over effects may include higher revenue or lower cost for the second development projects vis-‡-vis the case when the two projects are developed as if they are independent or by two independent developers. This paper sets a basic optimal development timing model with only one developer, who will develop two homogenous projects. We further extend the model to incorporate heterogeneous projects. We find that the developer will abort the project when the demand is weak, and he will choose to develop single project when the demand curve is steep. In a market with flat demand curve, it will be more optimal economically for the developer to develop the two projects simultaneously. The positive portfolio effects shorten the time to wait to develop for the two projects, and the developer will prefer to undertake the two projects simultaneously.