The Franciscan church of La Peregrina in Sahagún was founded in the 12th century. It has suffered along its history many transformations and additions. The most important was executed in the 17th. The church was converted to the baroque style. The apse vaults were demolished and new timbrel vaults were built hiding the mudéjar windows. In the nave, the transversal arches which supported the modern roof were also demolished, interior buttresses built and new barrel timbrel vaults with lunettes were erected. At the crossing an oval dome was finally built. In the course of recent restoration work, the building will have a new use. The project pretends to recover the mudéjar apse with its windows giving light to the presbytery. This involves the demolition of the baroque vaults above the presbytery. This side of the dome will loose some buttressing and an expertise was required about the feasibility of the operation. In the present paper the structure of the church will be described and the analysis of the oval dome with and without the presbytery vaults explained.