Ms Royal 2. B.V. in the British Library, London, is a tenth-century Psalter from Winchester, possibly from Nunnaminster. On the last folios of this MS (189-190) were added in the late tenth century, miscellaneous computistical entries, which include the years ofChrist, the ages of the world, the ages and generations, the numbering and reckoning of years and the number of years from the Creation to the foundation of Rome. Two texts, the ‘De longitudine mundi’ (fol. 189) and ‘Longitudo, latitudo et altitudo templi et tabernaculum (sic)’ (fols. 189r and v) precede, and another, ‘De area Noe’ (fol. 189v) follows a short text entitled ‘De aedificatio (sic) ecclesie sancti Petri apostoli’ at folio 189v.With the exception of this last, all these texts are also found in a ninth-century MS, British Library, Cotton Vespasian B.VI (fols. 106-70). To the best of my knowledge, the ‘De aedificatio[ne]’ does not exist in any other manuscript, and there is no known source for it. The present note aims solely at bringing this text to light and providing an edition of it; an examination of its implications for English history and architecture will be provided elsewhere.