Development of society and information systems nowadays imposes the need for developing web applications in a short period of time in accordance with the set requirements concerning efficient utilization of the system. During web project planning, project managers are primarily focused on the time variable. Development of web applications on the basis of the conceptual model provides a better estimation of development effort and reduces possible occurrence of errors in development phase. So far, a number of methods for web application development on the basis of the conceptual model have been proposed, but only few of them implement effort estimation process for web applications. Most of the proposed methods have some drawbacks in that they require a large database of completed projects in order to perform new model assessment since they use predefined weighting factors to calculate development effort estimation. In recent years, the focus of the attention in the field of software development effort estimation has shifted to calculating the functional size of the projects, since they: (1) are based on the functionality that will be delivered to the end user and not on the artifacts created as a product of finished applications (eg. number of web pages, multimedia files, etc.); (2) can be measured in the early phases of development of web applications; (3) are based on standards that define the concept of the size and requirements for the purposes of assessing the size of the application; (4) are widely applicable in the industry. There are several standardized methods of calculating the functional size of the software, but the method that has recently experienced tremendous acceptance by researchers and development engineers is the COSMIC (Common Software Measurement International Consortium) method. The appeal of the COSMIC method can be contributed to the fact that it is very simple to use and implement, and very easy to learn. Combined approaches to estimate web application development effort on the basis of the conceptual models and functional size are rather hard to find in literature. The thesis defines a procedure to create conceptual models and methods of calculating functional size of web applications on the basis of conceptual models for the purposes of designing a model for calculation of web application development effort. The research resulted in a mathematical model, based on a conceptual model, which uses the calculated functional size of a web application to estimate the level of web application development effort represented as a time variable. In addition, within the thesis, a system for automatic web application development effort estimation was developed. The system actively collects the data necessary for model calculation from a data repository of the previously developed web applications by the proposed model. The system is called WADEES (Web Application Effort Estimation System) and it enables automatic calculation of the CFP (COSMIC Function Point) based on conceptual model and estimation of a new web application development effort. Verification of the model for development effort estimation for web applications is carried out on the basis of the proposed model and a database of previously developed web applications. As a result of the verification of the research, a prototype system to estimate development effort of web applications established on the basis of the conceptual model was created.