Physically this is big enough for a modest house. After all, many terraced houses are typically no more than 5m in width. However, bear in mind that local planning policies may well dictate that the house must have off-street parking and a back garden. The depth of the plot could be more of an issue than the width. In addition, much will depend on whether a house could be designed to fit in with the pattern of surrounding buildings, and whether it would give rise to any issues of overlooking or loss of light to adjoining houses. Talk to a local building designer or speak to a planning officer at your local council to see whether they think it’s feasible on your site.